Computer Science and Discrete Mathematics (CSDM)

Theoretical Computer Science and Discrete Mathematics

The singularity of symbolic matrices

Avi Wigderson
Herbert H. Maass Professor, School of Mathematics
February 9, 2016
The main object of study of this talk are matrices whose entries are linear forms in a set of formal variables (over some field). The main problem is determining if a given such matrix is invertible or singular (over the appropriate field of rational functions). As it happens, this problem has a dual life; when the underlying variables commute, and when they do not.

Bipartite perfect matching is in quasi-NC

Stephen Fenner
University of South Carolina
February 8, 2016
We show that the bipartite perfect matching problem is in $\textrm{quasi-}\textsf{NC}^2$. That is, it has uniform circuits of quasi-polynomial size and $O(\log^2 n)$ depth. Previously, only an exponential upper bound was known on the size of such circuits with poly-logarithmic depth. We obtain our result by an almost complete derandomization of the Isolation Lemma of Mulmuley, Vazirani, & Vazirani, used to yield an efficient randomized parallel algorithm for the bipartite perfect matching problem.

Constant-round interactive-proofs for delegating computations

Ron Rothblum
Massachusetts Institute of Technology
February 1, 2016
Interactive proofs have had a dramatic impact on Complexity Theory and Cryptography. In particular, the celebrated IP=PSPACE Theorem [LFKN92,Shamir92] allows an all-powerful but untrusted prover to convince a polynomial-time verifier of the validity of extremely complicated statements (as long as they can be evaluated using polynomial space). The interactive proof system designed for this purpose requires a polynomial number of communication rounds.

Toward the KRW conjecture: cubic lower bounds via communication complexity

Or Meir
University of Haifa
December 14, 2015
One of the major challenges of the research in circuit complexity is proving super-polynomial lower bounds for de-Morgan formulas. Karchmer, Raz, and Wigderson suggested to approach this problem by proving that formula complexity behaves "as expected" with respect to the composition of functions. They showed that this conjecture, if proved, would imply super-polynomial formula lower bounds.

Ramanujan coverings of graphs

Doron Puder
Member, School of Mathematics
December 8, 2015
Ramanujan graphs are optimal expander graphs, and their existence and construction have been the focus of much research during the last three decades. We prove that every bipartite Ramanujan graph has a $d$-covering which is also Ramanujan. This generalizes the $d = 2$ case, a recent major breakthrough in the subject due to Marcus, Spielman and Srivastava. The main tools we use are the Peter-Weyl theory in group representations, as well as the theory of interlacing polynomials. All notions will be explained. Joint work with Chris Hall and Will Sawin.

Bias vs low rank of polynomials with applications to list decoding and effective algebraic geometry

Abhishek Bhowmick
University of Texas at Austin
December 7, 2015
Let $f$ be a polynomial of degree $d$ in $n$ variables over a finite field $\mathbb{F}$. The polynomial is said to be unbiased if the distribution of $f(x)$ for a uniform input $x \in \mathbb{F}^n$ is close to the uniform distribution over $\mathbb{F}$, and is called biased otherwise. The polynomial is said to have low rank if it can be expressed as a composition of a few lower degree polynomials. Green and Tao [Contrib. Discrete Math 2009] and Kaufman and Lovett [FOCS 2008] showed that bias implies low rank for fixed degree polynomials over fixed prime fields.

Rigidity of random Toeplitz matrices with an application to depth three circuits

Avishay Tal
Member, School of Mathematics
December 1, 2015
Joint work with Oded Goldreich. We prove that random $n$-by-$n$ Toeplitz matrices over $GF(2)$ have rigidity $\Omega(n^3/(r^2 \log n))$ for rank $r > \sqrt{n}$, with high probability. This improves, for $r = o(n / \log n \log\log n)$, over the $\Omega( (n^2 / r) \cdot \log(n/r) )$ bound that is known for many explicit matrices.

Lower bounds on the size of semidefinite programming relaxations

David Steurer
Cornell University
November 30, 2015
We introduce a method for proving lower bounds on the efficacy of semidefinite programming (SDP) relaxations for combinatorial problems. In particular, we show that the cut, TSP, and stable set polytopes on $n$-vertex graphs are not the linear image of the feasible region of any SDP (i.e., any spectrahedron) of dimension less than $2^{n^{\delta}}$, for some constant $\delta > 0$. This result yields the first super-polynomial lower bounds on the semidefinite extension complexity of any explicit family of polytopes.