Blends have decent numerical properties

Abstract

A "blend" is a two-point Hermite interpolational polynomial, typically of quite high degree. This note shows that implementing them in a double Horner evaluation scheme has good backward error, and also shows that the Lebesgue constant for a balanced blend or nearly balanced blend on the interval [0,1] is bounded by 2, independently of the grade or degree of the approximation. On [-1,1], which is a more natural interval for comparison, it is of course unbounded, but grows only like 2√(m/π) where 2m+1 is the grade of approximation. I also show that the quadrature schemes for balanced blends amplify errors only by O( ln(m) ).