Sums of squares and quadratic persistence on real projective varieties

Grigoriy Blekherman, Rainer Sinn, Gregory G. Smith, Mauricio Velasco

Research output: Contribution to journalArticlepeer-review

1 Scopus citations

Abstract

We bound the Pythagoras number of a real projective subvariety: the smallest positive integer r such that every sum of squares of linear forms in its homogeneous coordinate ring is a sum of at most r 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.

Original languageEnglish
Pages (from-to)925-965
Number of pages41
JournalJournal of the European Mathematical Society
Volume24
Issue number3
DOIs
StatePublished - 2021
Externally publishedYes

Keywords

  • Convex algebraic geometry
  • Linear syzygies
  • Pythagoras number
  • Sums of squares

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