Barriers for rank methods in arithmetic complexity

Klim Efremenko, Ankit Garg, Rafael Oliveira, Avi Wigderson

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

12 Scopus citations

Abstract

Arithmetic complexity, the study of the cost of computing polynomials via additions and multiplications, is considered (for many good reasons) simpler to understand than Boolean complexity, namely computing Boolean functions via logical gates. And indeed, we seem to have significantly more lower bound techniques and results in arithmetic complexity than in Boolean complexity. Despite many successes and rapid progress, however, foundational challenges, like proving superpolynomial lower bounds on circuit or formula size for explicit polynomials, or super-linear lower bounds on explicit 3-dimensional tensors, remain elusive. At the same time (and possibly for similar reasons), we have plenty more excuses, in the form of “barrier results” for failing to prove basic lower bounds in Boolean complexity than in arithmetic complexity. E orts to find barriers to arithmetic lower bound techniques seem harder, and despite some attempts we have no excuses of similar quality for these failures in arithmetic complexity. This paper aims to add to this study. In this paper we address rank methods, which were long recognized as encompassing and abstracting almost all known arithmetic lower bounds to-date, including the most recent impressive successes. Rank methods (under the name of flattenings) are also in wide use in algebraic geometry for proving tensor rank and symmetric tensor rank lower bounds. Our main results are barriers to these methods. In particular, Rank methods cannot prove better than d(nd/2) lower bound on the tensor rank of any d-dimensional tensor of side n. (In particular, they cannot prove super-linear, indeed even > 8n tensor rank lower bounds for any 3-dimensional tensors.) Rank methods cannot prove d(nd/2) on the Waring rank1 of any n-variate polynomial of degree d. (In particular, they cannot prove such lower bounds on stronger models, including depth-3 circuits.) The proofs of these bounds use simple linear-algebraic arguments, leveraging connections between the symbolic rank of matrix polynomials and the usual rank of their evaluations. These techniques can perhaps be extended to barriers for other arithmetic models on which progress has halted. To see how these barrier results directly inform the state-of-art in arithmetic complexity we note the following. First, the bounds above nearly match the best explicit bounds we know for these models, hence o er an explanations why the rank methods got stuck there. Second, the bounds above are a far cry (quadratically away) from the true complexity (e.g. of random polynomials) in these models, which if achieved (by any methods), are known to imply superpolynomial formula lower bounds. We also explain the relation of our barrier results to other attempts, and in particular how they significantly differ from the recent attempts to find analogues of “natural proofs” for arithmetic complexity. Finally, we discuss the few arithmetic lower bound approaches which fall outside rank methods, and some natural directions our barriers suggest.

Original languageEnglish
Title of host publication9th Innovations in Theoretical Computer Science, ITCS 2018
EditorsAnna R. Karlin
PublisherSchloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing
ISBN (Electronic)9783959770606
DOIs
StatePublished - 1 Jan 2018
Event9th Innovations in Theoretical Computer Science, ITCS 2018 - Cambridge, United States
Duration: 11 Jan 201814 Jan 2018

Publication series

NameLeibniz International Proceedings in Informatics, LIPIcs
Volume94
ISSN (Print)1868-8969

Conference

Conference9th Innovations in Theoretical Computer Science, ITCS 2018
Country/TerritoryUnited States
CityCambridge
Period11/01/1814/01/18

Keywords

  • Barriers
  • Flattenings
  • Lower Bounds
  • Partial Derivatives

Fingerprint

Dive into the research topics of 'Barriers for rank methods in arithmetic complexity'. Together they form a unique fingerprint.

Cite this