On the Complexity of Branching Proofs

Abstract

We consider the task of proving integer infeasibility of a bounded convex $K$ in $\mathbb{R}^n$ using a general branching proof system. In a general branching proof, one constructs a branching tree by adding an integer disjunction $\mathbf{a} \mathbf{x} \leq b$ or $\mathbf{a} \mathbf{x} \geq b+1$, $\mathbf{a} \in \mathbb{Z}^n$, $b \in \mathbb{Z}$, at each node, such that the leaves of the tree correspond to empty sets (i.e., $K$ together with the inequalities picked up from the root to leaf is empty). Recently, Beame et al (ITCS 2018), asked whether the bit size of the coefficients in a branching proof, which they named stabbing planes (SP) refutations, for the case of polytopes derived from SAT formulas, can be assumed to be polynomial in $n$. We resolve this question by showing that any branching proof can be recompiled so that the integer disjunctions have coefficients of size at most $(n R)^{O(n^2)}$, where $R \in \mathbb{N}$ such that $K \in R \mathbb{B}_1^n$, while increasing the number of nodes in the branching tree by at most a factor $O(n)$. As our second contribution, we show that Tseitin formulas, an important class of infeasible SAT instances, have quasi-polynomial sized cutting plane (CP) refutations, disproving the conjecture that Tseitin formulas are (exponentially) hard for CP. As our final contribution, we give a simple family of polytopes in $[0,1]^n$ requiring branching proofs of length $2^n/n$.