Mathematics – Algebraic Geometry
Scientific paper
2010-01-25
Mathematics
Algebraic Geometry
(v2) revision based on suggestions by referee
Scientific paper
10.1016/j.jpaa.2011.08.012
In this note we aim to give a new, elementary proof of a statement that was first proved by Timofte. It says that a symmetric real polynomial $F$ of degree $d$ in $n$ variables is positive on $\R^n$ (on $\R^{n}_{\geq 0}$) if and only if it is so on the subset of points with at most $\max\{\lfloor d/2\rfloor,2\}$ distinct components. We deduce Timofte's original statement as a corollary of a slightly more general statement on symmetric optimization problems. The idea we are using to prove this statement is to relate it to a linear optimization problem in the orbit space. The fact that for the case of the symmetric group $S_n$ this can be viewed as a question on normalized univariate real polynomials with only real roots allows us to conclude the theorems in a very elementary way. We hope that the methods presented here will make it possible to derive similar statements also in the case of other groups.
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