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Symmetry in Turán sums of squares polynomials from flag algebras. (English) Zbl 1388.05186
Summary: Turán problems in extremal combinatorics ask to find asymptotic bounds on the edge densities of graphs and hypergraphs that avoid specified subgraphs. The theory of flag algebras proposed by A. A. Razborov [J. Symb. Log. 72, No. 4, 1239–1282 (2007; Zbl 1146.03013)] provides powerful methods based on semidefinite programming to find sums of squares that establish edge density inequalities in Turán problems. Working with polynomial analogs of the flag algebra entities, we prove that such sums of squares created by flag algebras can be retrieved from a restricted version of the symmetry-adapted semidefinite program proposed by K. Gatermann and P. A. Parrilo [J. Pure Appl. Algebra 192, No. 1–3, 95–128 (2004; Zbl 1108.13021)]. This involves using the representation theory of the symmetric group for finding succinct sums of squares expressions for invariant polynomials. The connection reveals several combinatorial and structural properties of flag algebra sums of squares, and offers new tools for Turán and other related problems.
##### MSC:
 05D99 Extremal combinatorics 12D15 Fields related with sums of squares (formally real fields, Pythagorean fields, etc.) 90C22 Semidefinite programming
Flagmatic
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