Turán type converse Markov inequalities in $L^q$ on a generalized Erőd class of convex domains

Abstract

P. Turán was the first to derive lower estimations on the uniform norm of the derivatives of polynomials $p$ of uniform norm $1$ on the interval I:=[-1,1] and the disk D:=$\{z\in C:|z|\le 1\}$, under the normalization condition that the zeroes of the polynomial p in question all lie in I or D, resp. Namely, in 1939 he proved that with n:=deg p tending to infinity, the precise growth order of the minimal possible derivative norm is $\sqrt{n}$ for I and n for D. Already the same year J. Erőd considered the problem on other domains. In his most general formulation, he extended Turán's order n result on D to a certain general class of piecewise smooth convex domains. Finally, a decade ago the growth order of the minimal possible norm of the derivative was proved to be n for all compact convex domains. Turán himself gave comments about the above oscillation question in $L^q$ norm on D. Nevertheless, till recently results were known only for I, D and so-called R-circular domains. Continuing our recent work, also here we investigate the Turán-Erőd problem on general classes of domains.

arXiv admin note: text overlap with arXiv:1512.08268