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Convergence of series of polynomials over different intervals

Source: KoMaL A. 883

June 11, 2024
algebrapolynomialanalysisseries

Problem Statement

Let JIRJ\subsetneq I\subseteq \mathbb R be non-empty open intervals, and let f1,f2,f_1, f_2,\ldots be real polynomials satisfying the following conditions:
[*] fi(x)0f_i(x)\ge 0 for all i1i\ge 1 and xIx\in I, [*] i=1fi(x)\sum\limits_{i=1}^\infty f_i(x) is finite for all xIx\in I, [*] i=1fi(x)=1\sum\limits_{i=1}^\infty f_i(x)=1 for all xJx\in J.
Do these conditions imply that i=1fi(x)=1\sum\limits_{i=1}^\infty f_i(x)=1 also for all xIx\in I?
Proposed by András Imolay, Budapest