Abstract
This chapter gives a detailed discussion of barycentric Lagrange and Hermite interpolation and extends this to rational interpolation with a specified denominator. We discuss the conditioning of these interpolants. A numerically stable method to find roots of polynomials expressed in barycentric form via a generalized eigenvalue problem is given. We conclude with a section on piecewise polynomial interpolants. ⊲
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Notes
- 1.
Note the different use of “Lagrange interpolating polynomial” and “Lagrange basis polynomials.”
- 2.
A useful fact, from the numerical point of view, is that we may scale all \(\beta _{k}\) by any common factor. This can prevent unnecessary overflow or underflow.
- 3.
See also the forthcoming paper Stability of rootfinding for barycentric Lagrange enterpolants by Lawrence et al. (2013).
- 4.
This series needs to be convergent for these manipulations to be valid. It is, for z close enough to τ ℓ —in fact, for all z closer to τ ℓ than t is. Since t is different from each node by hypothesis, this is possible.
- 5.
Or by more efficient ones such as are described in van Deun et al. (2008).
- 6.
Also, as Gerhard Wanner tells us, Runge used contour integrals too in his celebrated 1901 paper for just this purpose.
- 7.
Some people might confuse this problem with the famous Wilkinson polynomial \(W(z) = (z - 1)(z - 2)\cdots (z - 20)\). Don’t. This problem has nothing to do with that polynomial.
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Corless, R.M., Fillion, N. (2013). Polynomial and Rational Interpolation. In: A Graduate Introduction to Numerical Methods. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8453-0_8
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