Sums of squares and quadratic persistence on real projective varieties
Grigoriy Blekherman
Georgia Tech, Atlanta, USARainer Sinn
Universität Leipzig, GermanyGregory G. Smith
Queen's University, Kingston, CanadaMauricio Velasco
Universidad de los Andes, Bogotá, Colombia
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Abstract
We bound the Pythagoras number of a real projective subvariety: the smallest positive integer such that every sum of squares of linear forms in its homogeneous coordinate ring is a sum of at most squares. Enhancing existing methods, we exhibit three distinct upper bounds involving known invariants. In contrast, our lower bound depends on a new invariant of a projective subvariety called the quadratic persistence. Defined by projecting away from points, this numerical invariant is closely related to the linear syzygies of the variety. In addition, we classify the projective subvarieties of maximal and almost-maximal quadratic persistence, and determine their Pythagoras numbers.
Cite this article
Grigoriy Blekherman, Rainer Sinn, Gregory G. Smith, Mauricio Velasco, Sums of squares and quadratic persistence on real projective varieties. J. Eur. Math. Soc. 24 (2022), no. 3, pp. 925–965
DOI 10.4171/JEMS/1108