CNF Encodings of Parity

Abstract

The minimum number of clauses in a CNF representation of the parity function $x_1\oplus x_2\oplus \cdots \oplus x_n$ is $2^{n-1}$. One can obtain a more compact CNF encoding by using non-deterministic variables (also known as guess or auxiliary variables). In this paper, we prove the following lower bounds, that almost match known upper bounds, on the number $m$ of clauses and the maximum width $k$ of clauses: 1) if there are at most $s$ auxiliary variables, then $m\ge \Omega \left(2^{n/(s+1)}/n\right)$ and $k\ge n/(s+1)$; 2) the minimum number of clauses is at least $3n$. We derive the first two bounds from the Satisfiability Coding Lemma due to Paturi, Pudlak, and Zane.