[SciPy-User] help interpreting univariate spline

Elliot Hallmark permafacture@gmail....
Fri Apr 23 18:53:13 CDT 2010


Hi there,

I am wanting to use a scipy interpolation routine to generate
polynomials approximating function I have sampled.  these polynomials
will be solved in c (cython), so I need to extract the coefficencts
from the interpolation method and pass them to a c function for root
finding.

using UnivariateSpline I ran this code:

import numpy as np
from scipy.interpolate import splrep, UnivariateSpline

x = np.linspace(1, 10, 100)
y = 1/x     #approximate a hyperbola
g = UnivariateSpline(x, y, w=None, k=3, s=.0005)
print g.get_knots()
print g.get_coeffs()

import matplotlib.pyplot as plt
plt.clf()
ax = plt.subplot(121)
l1, = ax.plot(x, y, "rs", ms=10)
l2, = ax.plot(x, g(x), "bo")
plt.show()




and the output was:
[  1.           2.18181818   3.27272727   5.54545455  10.        ]

[ 0.98462432  0.66575572  0.423       0.25461626  0.13929847  0.11763985
  0.09929961]

That is 5 knots and 7 coefficients for a degree=3 spline.  naively, I
was expecting 4 coefficents for each interval between knots.

how can I construct a piecewise polynomial from this output? or can I?

thx,

Elliot


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