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TR15-053 | 7th April 2015 03:03
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#### Tight Size-Degree Bounds for Sums-of-Squares Proofs

TR15-053
Authors:

Massimo Lauria,

Jakob Nordström
Publication: 7th April 2015 10:43

Downloads: 1403

Keywords:

clique,

degree,

Lasserre,

lower bound,

Positivstellensatz,

Rank,

Resolution,

Semidefinite programming,

size,

SOS,

sums-of-squares
**Abstract:**
We exhibit families of 4-CNF formulas over n variables that have sums-of-squares (SOS) proofs of unsatisfiability of degree (a.k.a. rank) d but require SOS proofs of size n^{Omega(d)} for values of d = d(n) from constant all the way up to n^{delta} for some universal constant delta. This shows that the n^{O(d)} running time obtained by using the Lasserre semidefinite programming relaxations to find degree-d SOS proofs is optimal up to constant factors in the exponent. We establish this result by combining NP-reductions expressible as low-degree SOS derivations with the idea of relativizing CNF formulas in [Krajicek '04] and [Dantchev and Riis '03], and then applying a restriction argument as in [Atserias, Müller, and Oliva '13] and [Atserias, Lauria, and Nordstrom '14]. This yields a generic method of amplifying SOS degree lower bounds to size lower bounds, and also generalizes the approach in [ALN14] to obtain size lower bounds for the proof systems resolution, polynomial calculus, and Sherali-Adams from lower bounds on width, degree, and rank, respectively.