complexity theory tool developers

Abstract: The sign rank of a ${\pm 1}$-valued matrix $A$ is the minimum rank of a real matrix that agrees with $A$ entrywise in sign. For every sufficiently large $d$, we give an explicit matrix $A$ with $2^{Θ(d^2)}$ rows and columns and sign rank $Θ(d)$ such that $A \land A$ has sign rank $2^{Θ(d)}$. Here, $A \land A$ is the communication matrix of the problem obtained by evaluating $A$ on disjoint inputs and taking the conjunction of the results. As the logarithm of sign rank characterizes unbounded-error communication complexity, this implies that the communication class $\mathsf{UPP}^{\mathsf{cc}}$ is not closed under intersection. The starting point for our lower bound is Forster's method, which relates the sign rank of a matrix to the average margin of representations of its rows and columns by points and halfspaces. We study a continuous analog for conjunctions of inner-product sign tests on the unit sphere. Rotational symmetry connects this problem to polynomial sign representations, yielding an exponentially small upper bound on the average margin. This bound holds even when sign constraints are relaxed near the sign tests' decision boundary, allowing us to transfer it to explicit finite sign matrices arising from intersections of integer halfspaces.

Abstract: We initiate the study of the meta-mathematics of algebraic circuit lower bounds, aiming both to gain insight into the methods sufficient and necessary to prove algebraic circuit lower bounds, and to contribute to the study of bounded arithmetic as a logical foundation for complexity lower bounds. We demonstrate that while algebraic circuit lower bounds are hard for somewhat weak proof systems such as Polynomial Calculus Resolution ($\mathsf{PCR}$), contemporary lower bounds are efficiently provable in proof systems and bounded arithmetic theories corresponding to $\mathsf{NC}^2$, such as $\mathsf{VNC}^2$ and the corresponding class of propositional Frege proofs of quasipolynomial-size. Moreover, going below $\mathsf{VNC}^2$ into algebraic constant-depth reasoning is likely insufficient to efficiently prove already constant-depth algebraic circuit lower bounds. Under an algebraic analogue of the Krajicek--Razborov conjecture, our results suggest that proving superpolynomial lower bounds for general algebraic circuits will require stronger reasoning than that used in the rank-based proofs studied here. Specifically, we show the following.