scipy.interpolate.

FloaterHormannInterpolator

class scipy.interpolate.FloaterHormannInterpolator(points, values, *, d=3)

Floater-Hormann barycentric rational interpolation.

As described in [1], the method of Floater and Hormann computes weights for a Barycentric rational interpolant with no poles on the real axis.

Parameters:

x1D array_like, shape (n,)

1-D array containing values of the independent variable. Values may be real or complex but must be finite.

yarray_like, shape (n, …)

Array containing values of the dependent variable. Infinite and NaN values of values and corresponding values of x will be discarded.

dint, optional

Blends n - d degree d polynomials together. For d = n - 1 it is equivalent to polynomial interpolation. Must satisfy 0 <= d < n, defaults to 3.

Attributes:

weightsarray

Weights of the barycentric approximation.

Methods

__call__(z)	Evaluate the rational approximation at given values.
poles()	Compute the poles of the rational approximation.
residues()	Compute the residues of the poles of the approximation.
roots()	Compute the zeros of the rational approximation.

See also

AAA

Barycentric rational approximation of real and complex functions.

pade

Padé approximation.

Notes

The Floater-Hormann interpolant is a rational function that interpolates the data with approximation order \(O(h^{d+1})\). The rational function blends n - d polynomials of degree d together to produce a rational interpolant that contains no poles on the real axis, unlike AAA. The interpolant is given by

\[r(x) = \frac{\sum_{i=0}^{n-d} \lambda_i(x) p_i(x)} {\sum_{i=0}^{n-d} \lambda_i(x)},\]

where \(p_i(x)\) is an interpolating polynomials of at most degree d through the points \((x_i,y_i),\dots,(x_{i+d},y_{i+d}), and :math:\)lambda_i(z)` are blending functions defined by

\[\lambda_i(x) = \frac{(-1)^i}{(x - x_i)\cdots(x - x_{i+d})}.\]

When d = n - 1 this reduces to polynomial interpolation.

Due to its stability following barycentric representation of the above equation is used instead for computation

\[r(z) = \frac{\sum_{k=1}^m\ w_k f_k / (x - x_k)}{\sum_{k=1}^m w_k / (x - x_k)},\]

where the weights \(w_j\) are computed as

\[\begin{split}w_k &= (-1)^{k - d} \sum_{i \in J_k} \prod_{j = i, j \neq k}^{i + d} 1/|x_k - x_j|, \\ J_k &= \{ i \in I: k - d \leq i \leq k\},\\ I &= \{0, 1, \dots, n - d\}.\end{split}\]

References

[1]

M.S. Floater and K. Hormann, “Barycentric rational interpolation with no poles and high rates of approximation”, Numer. Math. 107, 315 (2007). DOI:10.1007/s00211-007-0093-y

Examples

Try it in your browser!

Here we compare the method against polynomial interpolation for an example where the polynomial interpolation fails due to Runge’s phenomenon.

>>> import numpy as np
>>> from scipy.interpolate import (FloaterHormannInterpolator,
...                                BarycentricInterpolator)
>>> def f(z):
...     return 1/(1 + z**2)
>>> z = np.linspace(-5, 5, num=15)
>>> r = FloaterHormannInterpolator(z, f(z))
>>> p = BarycentricInterpolator(z, f(z))
>>> zz = np.linspace(-5, 5, num=1000)
>>> import matplotlib.pyplot as plt
>>> fig, ax = plt.subplots()
>>> ax.plot(zz, r(zz), label="Floater=Hormann")
>>> ax.plot(zz, p(zz), label="Polynomial")
>>> ax.legend()
>>> plt.show()

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