Inequalities for ultraspherical polynomials. Proof of a conjecture of I. Ra\c{s}a

Abstract

A recent conjecture by I. Ra\c{s}a asserts that the sum of the squared Bernstein basis polynomials is a convex function in $[0,1]$. This conjecture turns out to be equivalent to a certain upper pointwise estimate of the ratio $P_n^{\prime}(x)/P_n(x)$ for $x\geq 1$, where $P_n$ is the $n$-th Legendre polynomial. Here, we prove both upper and lower pointwise estimates for the ratios $\big(P_n^{(\lambda)}(x)\big)^{\prime}/P_n^{(\lambda)}(x)$, $~x\geq 1$, where $P_n^{(\lambda)}$ is the $n$-th ultraspherical polynomial. In particular, we validate Ra\c{s}a's conjecture.