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1208 lines
32 KiB
Python
1208 lines
32 KiB
Python
#-------------------------------------------------------------------------------
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# Name: polynomial
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# Purpose: Functions to operate on polynomials.
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#
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# Author: pab
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# polyXXX functions are based on functions found in the matlab toolbox polyutil written by
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# Author: Peter J. Acklam
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# E-mail: pjacklam@online.no
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# WWW URL: http://home.online.no/~pjacklam
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#
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# Created: 30.12.2008
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# Copyright: (c) pab 2008
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# Licence: LGPL
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#-------------------------------------------------------------------------------
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#!/usr/bin/env python
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"""
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Extended functions to operate on polynomials
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"""
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import warnings
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import numpy as np
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from numpy.lib.polynomial import *
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__all__ = np.lib.polynomial.__all__
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__all__ = __all__ + ['polyreloc', 'polyrescl', 'polytrim', 'poly2hstr', 'poly2str',
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'polyshift', 'polyishift', 'map_from_intervall', 'map_to_intervall',
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'cheb2poly', 'chebextr', 'chebroot', 'chebpoly', 'chebfit', 'chebval',
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'chebder', 'chebint', 'Cheb1d', 'dct', 'idct']
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def polyreloc(p,x,y=0.0):
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"""
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Relocate polynomial
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The polynomial `p` is relocated by "moving" it `x`
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units along the x-axis and `y` units along the y-axis.
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So the polynomial `r` is relative to the point (x,y) as
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the polynomial `p` is relative to the point (0,0).
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Parameters
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----------
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p : array-like, poly1d
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vector or matrix of column vectors of polynomial coefficients to relocate.
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(Polynomial coefficients are in decreasing order.)
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x : scalar
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distance to relocate P along x-axis
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y : scalar
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distance to relocate P along y-axis (default 0)
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Returns
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-------
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r : ndarray, poly1d
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vector/matrix/poly1d of relocated polynomial coefficients.
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See also
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--------
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polyrescl
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Example
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-------
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>>> import numpy as np
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>>> p = np.arange(6); p.shape = (2,-1)
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>>> np.polyval(p,0)
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array([3, 4, 5])
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>>> np.polyval(p,1)
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array([3, 5, 7])
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>>> r = polyreloc(p,-1) # move to the left along x-axis
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>>> np.polyval(r,-1) # = polyval(p,0)
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array([3, 4, 5])
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>>> np.polyval(r,0) # = polyval(p,1)
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array([3, 5, 7])
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"""
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truepoly = isinstance(p, poly1d)
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r = np.atleast_1d(p).copy()
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n = r.shape[0]
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# Relocate polynomial using Horner's algorithm
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for ii in range(n,1,-1):
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for i in range(1,ii):
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r[i] = r[i] - x*r[i-1]
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r[-1] = r[-1] + y
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if r.ndim>1 and r.shape[-1]==1:
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r.shape = (r.size,)
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if truepoly:
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r = poly1d(r)
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return r
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def polyrescl(p, x, y=1.0):
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"""
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Rescale polynomial.
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Parameters
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----------
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p : array-like, poly1d
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vector or matrix of column vectors of polynomial coefficients to rescale.
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(Polynomial coefficients are in decreasing order.)
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x,y : scalars
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defining the factors to rescale the polynomial `p` in
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x-direction and y-direction, respectively.
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Returns
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-------
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r : ndarray, poly1d
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vector/matrix/poly1d of rescaled polynomial coefficients.
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See also
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--------
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polyreloc
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Example
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-------
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>>> import numpy as np
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>>> p = np.arange(6); p.shape = (2,-1)
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>>> np.polyval(p,0)
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array([3, 4, 5])
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>>> np.polyval(p,1)
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array([3, 5, 7])
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>>> r = polyrescl(p,2) # scale by 2 along x-axis
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>>> np.polyval(r,0) # = polyval(p,0)
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array([ 3., 4., 5.])
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>>> np.polyval(r,2) # = polyval(p,1)
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array([ 3., 5., 7.])
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"""
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truepoly = isinstance(p, poly1d)
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r = np.atleast_1d(p).copy()
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n = r.shape[0]
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xscale =(float(x)**np.arange(1-n , 1))
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if r.ndim==1:
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q = y*r*xscale
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else:
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q = y*r*xscale[:,np.newaxis]
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if truepoly:
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q = poly1d(q)
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return q
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def polytrim(p):
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"""
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Trim polynomial by stripping off leading zeros.
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Parameters
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----------
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p : array-like, poly1d
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vector of polynomial coefficients in decreasing order.
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Returns
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-------
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r : ndarray, poly1d
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vector/matrix/poly1d of trimmed polynomial coefficients.
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Example
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-------
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>>> p = [0,1,2]
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>>> polytrim(p)
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array([1, 2])
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"""
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truepoly = isinstance(p, poly1d)
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if truepoly:
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return p
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else:
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r = np.atleast_1d(p).copy()
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# Remove leading zeros
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is_not_lead_zeros =np.logical_or.accumulate(r != 0,axis=0)
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if r.ndim==1:
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r = r[is_not_lead_zeros]
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else:
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is_not_lead_zeros = np.any(is_not_lead_zeros,axis=1)
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r = r[is_not_lead_zeros,:]
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return r
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def poly2hstr(p, variable='x' ):
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"""
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Return polynomial as a Horner represented string.
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Parameters
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----------
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p : array-like poly1d
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vector of polynomial coefficients in decreasing order.
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variable : string
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display character for variable
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Returns
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-------
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p_str : string
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consisting of the polynomial coefficients in the vector P multiplied
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by powers of the given `variable`.
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Examples
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--------
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>>> poly2hstr([1, 1, 2], 's' )
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'(s + 1)*s + 2'
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See also
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--------
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poly2str
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"""
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var = variable
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coefs = polytrim(np.atleast_1d(p))
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order = len(coefs)-1 # Order of polynomial.
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s = '' # Initialize output string.
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ix = 1;
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for expon in range(order,-1,-1):
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coef = coefs[order-expon]
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#% There is no point in adding a zero term (except if it's the only
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#% term, but we'll take care of that later).
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if coef == 0:
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ix = ix+1
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else:
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#% Append exponent if necessary.
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if ix>1:
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exponstr = '%.0f' % ix
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s = '%s**%s' % (s,exponstr);
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ix = 1
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#% Is it the first term?
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isfirst = s == ''
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# We need the coefficient only if it is different from 1 or -1 or
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# when it is the constant term.
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needcoef = (( abs(coef) != 1 ) | ( expon == 0 ) & isfirst) | 1-isfirst
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# We need the variable except in the constant term.
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needvar = ( expon != 0 )
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#% Add sign, but we don't need a leading plus-sign.
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if isfirst:
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if coef < 0:
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s = '-' # % Unary minus.
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else:
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if coef < 0:
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s = '%s - ' % s # % Binary minus (subtraction).
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else:
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s = '%s + ' % s # % Binary plus (addition).
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#% Append the coefficient if it is different from one or when it is
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#% the constant term.
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if needcoef:
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coefstr = '%.20g' % abs(coef)
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s = '%s%s' % (s,coefstr)
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#% Append variable if necessary.
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if needvar:
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#% Append a multiplication sign if necessary.
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if needcoef:
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if 1-isfirst:
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s = '(%s)' % s
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s = '%s*' % s
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s = '%s%s' % ( s, var )
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#% Now treat the special case where the polynomial is zero.
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if s=='':
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s = '0'
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return s
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def poly2str(p,variable='x'):
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"""
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Return polynomial as a string.
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Parameters
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----------
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p : array-like poly1d
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vector of polynomial coefficients in decreasing order.
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variable : string
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display character for variable
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Returns
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-------
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p_str : string
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consisting of the polynomial coefficients in the vector P multiplied
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by powers of the given `variable`.
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See also
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--------
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poly2hstr
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Examples
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--------
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>>> poly2str([1, 1, 2], 's' )
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's**2 + s + 2'
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"""
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thestr = "0"
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var = variable
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# Remove leading zeros
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coeffs = polytrim(np.atleast_1d(p))
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N = len(coeffs)-1
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for k in range(len(coeffs)):
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coefstr = '%.4g' % abs(coeffs[k])
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if coefstr[-4:] == '0000':
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coefstr = coefstr[:-5]
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power = (N-k)
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if power == 0:
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if coefstr != '0':
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newstr = '%s' % (coefstr,)
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else:
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if k == 0:
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newstr = '0'
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else:
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newstr = ''
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elif power == 1:
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if coefstr == '0':
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newstr = ''
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elif coefstr == 'b' or coefstr == '1':
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newstr = var
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else:
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newstr = '%s*%s' % (coefstr, var)
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else:
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if coefstr == '0':
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newstr = ''
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elif coefstr == 'b' or coefstr == '1':
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newstr = '%s**%d' % (var, power,)
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else:
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newstr = '%s*%s**%d' % (coefstr, var, power)
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if k > 0:
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if newstr != '':
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if coeffs[k] < 0:
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thestr = "%s - %s" % (thestr, newstr)
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else:
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thestr = "%s + %s" % (thestr, newstr)
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elif (k == 0) and (newstr != '') and (coeffs[k] < 0):
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thestr = "-%s" % (newstr,)
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else:
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thestr = newstr
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return thestr
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def polyshift(py,a=-1,b=1):
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"""
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Polynomial coefficient shift
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Polyshift shift the polynomial coefficients by a variable shift:
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Y = 2*(X-.5*(b+a))/(b-a)
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i.e., the interval -1 <= Y <= 1 is mapped to the interval a <= X <= b
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Parameters
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----------
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py : array-like
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polynomial coefficients for the variable y.
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a,b : scalars
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lower and upper limit.
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Returns
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-------
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px : ndarray
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polynomial coefficients for the variable x.
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See also
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--------
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polyishift
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Example
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-------
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>>> py = [1, 0]
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>>> px = polyshift(py,0,5)
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>>> polyval(px,[0, 2.5, 5]) #% This is the same as the line below
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array([-1., 0., 1.])
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>>> polyval(py,[-1, 0, 1 ])
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array([-1, 0, 1])
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"""
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if (a==-1) & (b ==1):
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return py
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L = b-a
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return polyishift(py,-(2.+b+a)/L,(2.-b-a)/L)
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def polyishift(px,a=-1,b=1):
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"""
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Inverse polynomial coefficient shift
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Polyishift does the inverse of Polyshift,
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shift the polynomial coefficients by a variable shift:
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Y = 2*(X-.5*(b+a)/(b-a)
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i.e., the interval a <= X <= b is mapped to the interval -1 <= Y <= 1
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Parameters
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----------
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px : array-like
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polynomial coefficients for the variable x.
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a,b : scalars
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lower and upper limit.
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Returns
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-------
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py : ndarray
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polynomial coefficients for the variable y.
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See also
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--------
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polyishift
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Example
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-------
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>>> px = [1, 0]
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>>> py = polyishift(px,0,5);
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>>> polyval(px,[0, 2.5, 5]) #% This is the same as the line below
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array([ 0. , 2.5, 5. ])
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>>> polyval(py,[-1, 0, 1])
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array([ 0. , 2.5, 5. ])
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"""
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if (a==-1) & (b ==1):
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return px
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L = b-a
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xscale = 2./L
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xloc = -float(a+b)/L
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return polyreloc( polyrescl(px,xscale),xloc)
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def map_from_interval(x,a,b) :
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"""F(x), where F: [a,b] -> [-1,1]."""
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return (x - (b + a)/2.0)*(2.0/(b - a))
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def map_to_interval(x,a,b) :
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"""F(x), where F: [-1,1] -> [a,b]."""
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return (x*(b - a) + (b + a))/2.0
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def poly2cheb(p,a=-1,b=1):
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"""
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Convert polynomial coefficients into Chebyshev coefficients
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Parameters
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----------
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p : array-like
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polynomial coefficients
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a,b : real scalars
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lower and upper limits (Default -1,1)
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Returns
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-------
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ck : ndarray
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Chebychef coefficients
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POLY2CHEB do the inverse of CHEB2POLY: given a vector of polynomial
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coefficients AK, returns an equivalent vector of Chebyshev
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coefficients CK.
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This is useful for economization of power series.
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The steps for doing so:
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1. Convert polynomial coefficients to Chebychev coefficients, CK.
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2. Truncate the CK series to a smaller number of terms, using the
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coefficient of the first neglected Chebychev polynomial as an error
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estimate.
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3 Convert back to a polynomial by CHEB2POLY
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See also
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--------
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cheb2poly
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chebval
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chebfit
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Examples
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--------
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>>> import numpy as np
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>>> p = np.arange(5)
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>>> ck = poly2cheb(p)
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>>> cheb2poly(ck)
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array([ 1., 2., 3., 4.])
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Reference
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---------
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William H. Press, Saul Teukolsky,
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William T. Wetterling and Brian P. Flannery (1997)
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"Numerical recipes in Fortran 77", Vol. 1, pp 184-194
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"""
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f = poly1d(p)
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n = len(f.coeffs)
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return chebfit(f,n,a,b)
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def cheb2poly(ck,a=-1,b=1):
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"""
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Converts Chebyshev coefficients to polynomial coefficients
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Parameters
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----------
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ck : array-like
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Chebychef coefficients
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a,b : real, scalars
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lower and upper limits (Default -1,1)
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Returns
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-------
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p : ndarray
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polynomial coefficients
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It is not advised to do this for len(ck)>10 due to numerical cancellations.
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See also
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--------
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chebval
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chebfit
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Examples
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--------
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>>> import numpy as np
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>>> p = np.arange(5)
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>>> ck = poly2cheb(p)
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>>> cheb2poly(ck)
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array([ 1., 2., 3., 4.])
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References
|
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----------
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http://en.wikipedia.org/wiki/Chebyshev_polynomials
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http://en.wikipedia.org/wiki/Chebyshev_form
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http://en.wikipedia.org/wiki/Clenshaw_algorithm
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"""
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n = len(ck)
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b_Nmi = np.zeros(1)
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b_Nmip1 = np.zeros(1)
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y = np.r_[2/(b-a), -(a+b)/(b-a)]
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y2 = 2.*y
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# Clenshaw recurence
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for ix in range(n-1,0,-1):
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tmp = b_Nmi
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b_Nmi = polymul(y2,b_Nmi) # polynomial multiplication
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nb = len(b_Nmip1)
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b_Nmip1[-1] = b_Nmip1[-1]-ck[ix]
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b_Nmi[-nb::] = b_Nmi[-nb::]-b_Nmip1
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b_Nmip1 = tmp
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p = polymul(y,b_Nmi) # polynomial multiplication
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nb = len(b_Nmip1)
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b_Nmip1[-1] = b_Nmip1[-1]-ck[0]
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p[-nb::] = p[-nb::]-b_Nmip1
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return polytrim(p)
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def chebextr(n):
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"""
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Return roots of derivative of Chebychev polynomial of the first kind.
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All local extreme values of the polynomial are either -1 or 1. So,
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CHEBPOLY( N, CHEBEXTR(N) ) ) return the same as (-1).^(N:-1:0)
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except for the numerical noise in the former.
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Because the extreme values of Chebychev polynomials of the first
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kind are either -1 or 1, their roots are often used as starting
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values for the nodes in minimax approximations.
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Parameters
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----------
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n : scalar, integer
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degree of Chebychev polynomial.
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Examples
|
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--------
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>>> x = chebextr(4)
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>>> chebpoly(4,x)
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array([ 1., -1., 1., -1., 1.])
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|
|
|
Reference
|
|
---------
|
|
http://en.wikipedia.org/wiki/Chebyshev_nodes
|
|
http://en.wikipedia.org/wiki/Chebyshev_polynomials
|
|
"""
|
|
return -np.cos((np.pi*np.arange(n+1))/n);
|
|
|
|
def chebroot(n,kind=1):
|
|
"""
|
|
Return roots of Chebychev polynomial of the first or second kind.
|
|
|
|
The roots of the Chebychev polynomial of the first kind form a particularly
|
|
good set of nodes for polynomial interpolation because the resulting
|
|
interpolation polynomial minimizes the problem of Runge's phenomenon.
|
|
|
|
Parameters
|
|
----------
|
|
n : scalar, integer
|
|
degree of Chebychev polynomial.
|
|
kind: 1 or 2, optional
|
|
kind of Chebychev polynomial (default 1)
|
|
|
|
Examples
|
|
--------
|
|
>>> import numpy as np
|
|
>>> x = chebroot(3)
|
|
>>> np.abs(chebpoly(3,x))<1e-15
|
|
array([ True, True, True], dtype=bool)
|
|
>>> chebpoly(3)
|
|
array([ 4., 0., -3., 0.])
|
|
>>> x2 = chebroot(4,kind=2)
|
|
>>> np.abs(chebpoly(4,x2,kind=2))<1e-15
|
|
array([ True, True, True, True], dtype=bool)
|
|
>>> chebpoly(4,kind=2)
|
|
array([ 16., 0., -12., 0., 1.])
|
|
|
|
|
|
Reference
|
|
---------
|
|
http://en.wikipedia.org/wiki/Chebyshev_nodes
|
|
http://en.wikipedia.org/wiki/Chebyshev_polynomials
|
|
"""
|
|
if kind not in (1,2):
|
|
raise ValueError('kind must be 1 or 2')
|
|
return -np.cos(np.pi*(np.arange(n)+0.5*kind)/(n+kind-1));
|
|
|
|
|
|
def chebpoly(n, x=None, kind=1):
|
|
"""
|
|
Return Chebyshev polynomial of the first or second kind.
|
|
|
|
These polynomials are orthogonal on the interval [-1,1], with
|
|
respect to the weight function w(x) = (1-x^2)^(-1/2+kind-1).
|
|
|
|
chebpoly(n) returns the coefficients of the Chebychev polynomial of degree N.
|
|
chebpoly(n,x) returns the Chebychev polynomial of degree N evaluated in X.
|
|
|
|
Parameters
|
|
----------
|
|
n : integer, scalar
|
|
degree of Chebychev polynomial.
|
|
x : array-like, optional
|
|
evaluation points
|
|
kind: 1 or 2, optional
|
|
kind of Chebychev polynomial (default 1)
|
|
|
|
Returns
|
|
-------
|
|
p : ndarray
|
|
polynomial coefficients if x is None.
|
|
Chebyshev polynomial evaluated at x otherwise
|
|
|
|
Examples
|
|
--------
|
|
>>> import numpy as np
|
|
>>> x = chebroot(3)
|
|
>>> np.abs(chebpoly(3,x))<1e-15
|
|
array([ True, True, True], dtype=bool)
|
|
>>> chebpoly(3)
|
|
array([ 4., 0., -3., 0.])
|
|
>>> x2 = chebroot(4,kind=2)
|
|
>>> np.abs(chebpoly(4,x2,kind=2))<1e-15
|
|
array([ True, True, True, True], dtype=bool)
|
|
>>> chebpoly(4,kind=2)
|
|
array([ 16., 0., -12., 0., 1.])
|
|
|
|
|
|
Reference
|
|
---------
|
|
http://en.wikipedia.org/wiki/Chebyshev_polynomials
|
|
"""
|
|
if x is None: # Calculate coefficients.
|
|
if n == 0:
|
|
p = np.ones(1)
|
|
else:
|
|
p = np.round( pow(2,n-2+kind) * np.poly( chebroot(n,kind=kind) ) )
|
|
p[1::2] = 0;
|
|
return p
|
|
else: # Evaluate polynomial in chebychev form
|
|
ck = np.zeros(n+1)
|
|
ck[n] = 1.
|
|
return _chebval(np.atleast_1d(x),ck,kind=kind)
|
|
|
|
def chebfit(fun,n=10,a=-1,b=1,trace=False):
|
|
"""
|
|
Computes the Chebyshevs coefficients
|
|
|
|
so that f(x) can be approximated by:
|
|
|
|
n-1
|
|
f(x) = sum ck*Tk(x)
|
|
k=0
|
|
|
|
where Tk is the k'th Chebyshev polynomial of the first kind.
|
|
|
|
Parameters
|
|
----------
|
|
fun : callable
|
|
function to approximate
|
|
n : integer, scalar, optional
|
|
number of base points (abscissas). Default n=10 (maximum 50)
|
|
a,b : real, scalars, optional
|
|
integration limits
|
|
|
|
Returns
|
|
-------
|
|
ck : ndarray
|
|
polynomial coefficients in Chebychev form.
|
|
|
|
Examples
|
|
--------
|
|
Fit exp(x)
|
|
|
|
>>> import pylab as pb
|
|
>>> a = 0; b = 2
|
|
>>> ck = chebfit(pb.exp,7,a,b);
|
|
>>> x = pb.linspace(0,4);
|
|
>>> h=pb.plot(x,pb.exp(x),'r',x,chebval(x,ck,a,b),'g.')
|
|
>>> x1 = chebroot(9)*(b-a)/2+(b+a)/2
|
|
>>> ck1 = chebfit(pb.exp(x1))
|
|
>>> h=pb.plot(x,pb.exp(x),'r',x,chebval(x,ck1,a,b),'g.')
|
|
|
|
>>> pb.close()
|
|
|
|
See also
|
|
--------
|
|
chebval
|
|
|
|
Reference
|
|
---------
|
|
http://en.wikipedia.org/wiki/Chebyshev_nodes
|
|
http://mathworld.wolfram.com/ChebyshevApproximationFormula.html
|
|
|
|
W. Fraser (1965)
|
|
"A Survey of Methods of Computing Minimax and Near-Minimax Polynomial
|
|
Approximations for Functions of a Single Independent Variable"
|
|
Journal of the ACM (JACM), Vol. 12 , Issue 3, pp 295 - 314
|
|
|
|
William H. Press, Saul Teukolsky,
|
|
William T. Wetterling and Brian P. Flannery (1997)
|
|
"Numerical recipes in Fortran 77", Vol. 1, pp 184-194
|
|
"""
|
|
|
|
if (n>50):
|
|
warnings.warn('CHEBFIT should only be used for n<50')
|
|
|
|
if hasattr(fun,'__call__'):
|
|
x = map_to_interval(chebroot(n),a,b)
|
|
f = fun(x);
|
|
if trace:
|
|
import pylab as plb
|
|
plb.plot(x,f,'+')
|
|
else:
|
|
f = fun
|
|
n = len(f)
|
|
#raise ValueError('Function must be callable!')
|
|
# N-1
|
|
# c(k) = (2/N) sum w(n) f(n)*cos(pi*k*(2n+1)/(2N)), 0 <= k < N.
|
|
# n=0
|
|
#
|
|
# w(0) = 0.5, w(n)=1 for n>0
|
|
ck = dct(f[::-1])/n
|
|
ck[0] = ck[0]/2.
|
|
return ck
|
|
|
|
def dct(x,n=None):
|
|
"""
|
|
Discrete Cosine Transform
|
|
|
|
N-1
|
|
y[k] = 2* sum x[n]*cos(pi*k*(2n+1)/(2*N)), 0 <= k < N.
|
|
n=0
|
|
|
|
Examples
|
|
--------
|
|
>>> import numpy as np
|
|
>>> x = np.arange(5)
|
|
>>> np.abs(x-idct(dct(x)))<1e-14
|
|
array([ True, True, True, True, True], dtype=bool)
|
|
>>> np.abs(x-dct(idct(x)))<1e-14
|
|
array([ True, True, True, True, True], dtype=bool)
|
|
|
|
Reference
|
|
---------
|
|
http://en.wikipedia.org/wiki/Discrete_cosine_transform
|
|
http://users.ece.utexas.edu/~bevans/courses/ee381k/lectures/
|
|
"""
|
|
fft = np.fft.fft
|
|
x = np.atleast_1d(x)
|
|
|
|
if n is None:
|
|
n = x.shape[-1]
|
|
|
|
if x.shape[-1]<n:
|
|
n_shape = x.shape[:-1] + (n-x.shape[-1],)
|
|
xx = np.hstack((x,np.zeros(n_shape)))
|
|
else:
|
|
xx = x[...,:n]
|
|
|
|
real_x = np.all(np.isreal(xx))
|
|
if (real_x and (np.remainder(n,2) == 0)):
|
|
xp = 2 * fft(np.hstack( (xx[...,::2], xx[...,::-2]) ))
|
|
else:
|
|
xp = fft(np.hstack((xx, xx[...,::-1])))
|
|
xp = xp[...,:n]
|
|
|
|
w = np.exp(-1j * np.arange(n) * np.pi/(2*n))
|
|
|
|
y = xp*w
|
|
|
|
if real_x:
|
|
return y.real
|
|
else:
|
|
return y
|
|
|
|
def idct(x,n=None):
|
|
"""
|
|
Inverse Discrete Cosine Transform
|
|
|
|
N-1
|
|
x[k] = 1/N sum w[n]*y[n]*cos(pi*k*(2n+1)/(2*N)), 0 <= k < N.
|
|
n=0
|
|
|
|
w(0) = 1/2
|
|
w(n) = 1 for n>0
|
|
|
|
Examples
|
|
--------
|
|
>>> import numpy as np
|
|
>>> x = np.arange(5)
|
|
>>> np.abs(x-idct(dct(x)))<1e-14
|
|
array([ True, True, True, True, True], dtype=bool)
|
|
>>> np.abs(x-dct(idct(x)))<1e-14
|
|
array([ True, True, True, True, True], dtype=bool)
|
|
|
|
Reference
|
|
---------
|
|
http://en.wikipedia.org/wiki/Discrete_cosine_transform
|
|
http://users.ece.utexas.edu/~bevans/courses/ee381k/lectures/
|
|
"""
|
|
|
|
ifft = np.fft.ifft
|
|
x = np.atleast_1d(x)
|
|
|
|
if n is None:
|
|
n = x.shape[-1]
|
|
|
|
w = np.exp(1j * np.arange(n) * np.pi/(2*n))
|
|
|
|
if x.shape[-1]<n:
|
|
n_shape = x.shape[:-1] + (n-x.shape[-1],)
|
|
xx = np.hstack((x,np.zeros(n_shape)))*w
|
|
else:
|
|
xx = x[...,:n]*w
|
|
|
|
real_x = np.all(np.isreal(x))
|
|
if (real_x and (np.remainder(n,2) == 0)):
|
|
xx[...,0] = xx[...,0]*0.5
|
|
yp = ifft(xx)
|
|
y = np.zeros(xx.shape,dtype=complex)
|
|
y[...,::2] = yp[...,:n/2]
|
|
y[...,::-2] = yp[...,n/2::]
|
|
else:
|
|
yp = ifft(np.hstack((xx, np.zeros_like(xx[...,0]), np.conj(xx[...,:0:-1]))))
|
|
y = yp[...,:n]
|
|
|
|
if real_x:
|
|
return y.real
|
|
else:
|
|
return y
|
|
|
|
def _chebval(x,ck,kind=1):
|
|
"""
|
|
Evaluate polynomial in Chebyshev form.
|
|
|
|
A polynomial of degree N in Chebyshev form is a polynomial p(x) of the form:
|
|
|
|
N
|
|
p(x) = sum ck*Tk(x)
|
|
k=0
|
|
or
|
|
N
|
|
p(x) = sum ck*Uk(x)
|
|
k=0
|
|
|
|
where Tk and Uk are the k'th Chebyshev polynomial of the first and second
|
|
kind, respectively.
|
|
|
|
References
|
|
----------
|
|
http://en.wikipedia.org/wiki/Clenshaw_algorithm
|
|
http://mathworld.wolfram.com/ClenshawRecurrenceFormula.html
|
|
"""
|
|
n = len(ck)
|
|
b_Nmi = np.zeros(x.shape) # b_(N-i)
|
|
b_Nmip1 = b_Nmi.copy() # b_(N-i+1)
|
|
x2 = 2*x
|
|
# Clenshaw reccurence
|
|
for ix in range(n-1,0,-1):
|
|
tmp = b_Nmi
|
|
b_Nmi = x2*b_Nmi - b_Nmip1 + ck[ix]
|
|
b_Nmip1 = tmp
|
|
return kind*x*b_Nmi - b_Nmip1 + ck[0]
|
|
|
|
|
|
def chebval(x,ck,a=-1,b=1,kind=1,fill=None):
|
|
"""
|
|
Evaluate polynomial in Chebyshev form at X
|
|
|
|
A polynomial of degree N in Chebyshev form is a polynomial p(x) of the form:
|
|
|
|
N
|
|
p(x) = sum ck*Tk(x)
|
|
k=0
|
|
|
|
where Tk is the k'th Chebyshev polynomial of the first or second kind.
|
|
|
|
Paramaters
|
|
----------
|
|
x : array-like
|
|
points to evaluate
|
|
ck : array-like
|
|
polynomial coefficients in Chebyshev form.
|
|
a,b : real, scalars, optional
|
|
limits for polynomial (Default -1,1)
|
|
kind: 1 or 2, optional
|
|
kind of Chebychev polynomial (default 1)
|
|
fill : scalar, optional
|
|
If provided, define value to return for `x < a` or `b < x`.
|
|
|
|
Examples
|
|
--------
|
|
Plot Chebychev polynomial of the first kind and order 4:
|
|
>>> import pylab as pb
|
|
>>> x = pb.linspace(-1,1)
|
|
>>> ck = pb.zeros(5); ck[-1]=1
|
|
>>> h = pb.plot(x,chebval(x,ck),x,chebpoly(4,x),'.')
|
|
>>> pb.close()
|
|
|
|
Fit exponential function:
|
|
>>> import pylab as pb
|
|
>>> ck = chebfit(pb.exp,7,0,2)
|
|
>>> x = pb.linspace(0,4);
|
|
>>> h=pb.plot(x,chebval(x,ck,0,2),'g',x,pb.exp(x))
|
|
>>> pb.close()
|
|
|
|
See also
|
|
--------
|
|
chebfit
|
|
|
|
References
|
|
----------
|
|
http://en.wikipedia.org/wiki/Clenshaw_algorithm
|
|
http://mathworld.wolfram.com/ClenshawRecurrenceFormula.html
|
|
"""
|
|
|
|
y = map_from_interval(np.atleast_1d(x),a,b)
|
|
if fill is None:
|
|
f = _chebval(y,ck,kind=kind)
|
|
else:
|
|
cond = (np.abs(y)<=1)
|
|
f = np.where(cond,0,fill)
|
|
if np.any(cond):
|
|
yk = np.extract(cond,y)
|
|
f[cond] = _chebval(yk,ck,kind=kind)
|
|
return f
|
|
|
|
|
|
def chebder(ck,a=-1,b=1):
|
|
"""
|
|
Differentiate Chebyshev polynomial
|
|
|
|
Parameters
|
|
----------
|
|
ck : array-like
|
|
polynomial coefficients in Chebyshev form of function to differentiate
|
|
a,b : real, scalars
|
|
limits for polynomial(Default -1,1)
|
|
|
|
Return
|
|
------
|
|
cder : ndarray
|
|
polynomial coefficients in Chebyshev form of the derivative
|
|
|
|
Examples
|
|
--------
|
|
|
|
Fit exponential function:
|
|
>>> import pylab as pb
|
|
>>> ck = chebfit(pb.exp,7,0,2)
|
|
>>> x = pb.linspace(0,4)
|
|
>>> ck2 = chebder(ck,0,2);
|
|
>>> h=pb.plot(x,chebval(x,ck,0,2),'g',x,pb.exp(x),'r')
|
|
>>> pb.close()
|
|
|
|
See also
|
|
--------
|
|
chebint
|
|
chebfit
|
|
"""
|
|
|
|
n = len(ck)
|
|
cder = np.zeros(n)
|
|
# n and n-1 are special cases.
|
|
# cder(n-1)=0;
|
|
cder[-2] = 2*(n-1)*ck[-1]
|
|
for j in range(n-3,-1,-1):
|
|
cder[j] = cder[j+2]+2*j*ck[j+1]
|
|
|
|
return cder*2./(b-a) # Normalize to the interval b-a.
|
|
|
|
def chebint(ck,a=-1,b=1):
|
|
"""
|
|
Integrate Chebyshef polynomial
|
|
|
|
Parameters
|
|
----------
|
|
ck : array-like
|
|
polynomial coefficients in Chebyshev form of function to integrate.
|
|
a,b : real, scalars
|
|
limits for polynomial(Default -1,1)
|
|
|
|
Return
|
|
------
|
|
cint : ndarray
|
|
polynomial coefficients in Chebyshev form of the integrated function
|
|
|
|
Examples
|
|
--------
|
|
Fit exponential function:
|
|
>>> import pylab as pb
|
|
>>> ck = chebfit(pb.exp,7,0,2)
|
|
>>> x = pb.linspace(0,4)
|
|
>>> ck2 = chebint(ck,0,2);
|
|
>>> h=pb.plot(x,chebval(x,ck,0,2),'g',x,pb.exp(x),'r.')
|
|
>>> pb.close()
|
|
|
|
See also
|
|
--------
|
|
chebder
|
|
chebfit
|
|
"""
|
|
|
|
|
|
n = len(ck)
|
|
|
|
cint = np.zeros(n);
|
|
con = 0.25*(b-a);
|
|
|
|
dif1 = np.diff(ck[::2])
|
|
ix1= np.r_[1:n-1:2]
|
|
cint[ix1] = -(con*dif1)/(ix1-1)
|
|
if n>3:
|
|
dif2 = np.diff(ck[1::2])
|
|
ix2=np.r_[2:n-1:2]
|
|
cint[ix2] = -(con*dif2)/(ix2-1)
|
|
|
|
#% cint(n) is a special case
|
|
cint[-1] = (con*ck[n-2])/(n-1)
|
|
cint[0] = np.sum((-1)**np.r_[1:n]*cint[1::]) # Set constant of integration
|
|
|
|
|
|
return cint
|
|
|
|
class Cheb1d(object):
|
|
coeffs = None
|
|
order = None
|
|
a = None
|
|
b = None
|
|
def __init__(self,ck,a=-1,b=1):
|
|
if isinstance(ck, poly1d):
|
|
for key in ck.__dict__.keys():
|
|
self.__dict__[key] = ck.__dict__[key]
|
|
return
|
|
cki = np.trim_zeros(np.atleast_1d(ck),'b')
|
|
if len(cki.shape) > 1:
|
|
raise ValueError, "Polynomial must be 1d only."
|
|
self.__dict__['coeffs'] = cki
|
|
self.__dict__['order'] = len(cki) - 1
|
|
self.__dict__['a'] = a
|
|
self.__dict__['b'] = b
|
|
|
|
|
|
def __call__(self,x):
|
|
return chebval(x,self.coeffs,self.a,self.b)
|
|
|
|
def __array__(self, t=None):
|
|
if t:
|
|
return np.asarray(self.coeffs, t)
|
|
else:
|
|
return np.asarray(self.coeffs)
|
|
|
|
def __repr__(self):
|
|
vals = repr(self.coeffs)
|
|
vals = vals[6:-1]
|
|
return "Cheb1d(%s)" % vals
|
|
|
|
def __len__(self):
|
|
return self.order
|
|
|
|
def __str__(self):
|
|
pass
|
|
def __neg__(self):
|
|
return Cheb1d(-self.coeffs,self.a,self.b)
|
|
|
|
def __pos__(self):
|
|
return self
|
|
|
|
|
|
def __add__(self, other):
|
|
other = poly1d(other)
|
|
return poly1d(polyadd(self.coeffs, other.coeffs))
|
|
|
|
def __radd__(self, other):
|
|
other = poly1d(other)
|
|
return poly1d(polyadd(self.coeffs, other.coeffs))
|
|
|
|
def __sub__(self, other):
|
|
other = poly1d(other)
|
|
return poly1d(polysub(self.coeffs, other.coeffs))
|
|
|
|
def __rsub__(self, other):
|
|
other = poly1d(other)
|
|
return poly1d(polysub(other.coeffs, self.coeffs))
|
|
|
|
def __eq__(self, other):
|
|
return np.alltrue(self.coeffs == other.coeffs)
|
|
|
|
def __ne__(self, other):
|
|
return np.any(self.coeffs != other.coeffs) or (self.a!=other.a) or (self.b !=other.b)
|
|
|
|
def __setattr__(self, key, val):
|
|
raise ValueError, "Attributes cannot be changed this way."
|
|
|
|
def __getattr__(self, key):
|
|
if key in ['c','coef','coefficients']:
|
|
return self.coeffs
|
|
elif key in ['o']:
|
|
return self.order
|
|
elif key in ['a']:
|
|
return self.a
|
|
elif key in ['b']:
|
|
return self.b
|
|
else:
|
|
try:
|
|
return self.__dict__[key]
|
|
except KeyError:
|
|
raise AttributeError("'%s' has no attribute '%s'" % (self.__class__, key))
|
|
def __getitem__(self, val):
|
|
if val > self.order:
|
|
return 0
|
|
if val < 0:
|
|
return 0
|
|
return self.coeffs[val]
|
|
|
|
def __setitem__(self, key, val):
|
|
ind = self.order - key
|
|
if key < 0:
|
|
raise ValueError, "Does not support negative powers."
|
|
if key > self.order:
|
|
zr = NX.zeros(key-self.order, self.coeffs.dtype)
|
|
self.__dict__['coeffs'] = NX.concatenate((self.coeffs,zr))
|
|
self.__dict__['order'] = key
|
|
self.__dict__['coeffs'][key] = val
|
|
return
|
|
|
|
def __iter__(self):
|
|
return iter(self.coeffs)
|
|
def integ(self, m=1):
|
|
"""
|
|
Return an antiderivative (indefinite integral) of this polynomial.
|
|
|
|
Refer to `chebint` for full documentation.
|
|
|
|
See Also
|
|
--------
|
|
chebint : equivalent function
|
|
|
|
"""
|
|
return Cheb1d(chebint(self.coeffs, self.a,self.b))
|
|
|
|
def deriv(self, m=1):
|
|
"""
|
|
Return a derivative of this polynomial.
|
|
|
|
Refer to `chebder` for full documentation.
|
|
|
|
See Also
|
|
--------
|
|
chebder : equivalent function
|
|
|
|
"""
|
|
return Cheb1d(chebder(self.coeffs,self.a,self.b))
|
|
def main():
|
|
if False: #True: #
|
|
x = np.arange(4)
|
|
dx = dct(x)
|
|
idx = idct(dx)
|
|
import pylab as plb
|
|
a = 0;
|
|
b = 2;
|
|
ck = chebfit(np.exp,6,a,b);
|
|
t = chebval(0,ck,a,b)
|
|
x=np.linspace(0,2,6);
|
|
plb.plot(x,np.exp(x),'r', x,chebval(x,ck,a,b),'g.')
|
|
#x1 = chebroot(9).'*(b-a)/2+(b+a)/2 ;
|
|
#ck1 =chebfit([x1 exp(x1)],9,a,b);
|
|
#plot(x,exp(x),'r'), hold on
|
|
#plot(x,chebval(x,ck1,a,b),'g'), hold off
|
|
|
|
|
|
t = poly2hstr([1,1,2])
|
|
py = [1, 0]
|
|
px = polyshift(py,0,5);
|
|
t1=polyval(px,[0, 2.5, 5]) #% This is the same as the line below
|
|
t2=polyval(py,[-1, 0, 1 ])
|
|
|
|
px = [1, 0]
|
|
py = polyishift(px,0,5);
|
|
t1 = polyval(px,[0, 2.5, 5]) #% This is the same as the line below
|
|
t2 = polyval(py,[-1, 0, 1 ])
|
|
print(t1,t2)
|
|
else:
|
|
import doctest
|
|
doctest.testmod()
|
|
if __name__== '__main__':
|
|
main()
|