Regular expression length via arithmetic formula complexity (2012.15617v1)

Abstract: We prove lower bounds on the length of regular expressions for finite languages by methods from arithmetic circuit complexity. First, we show a reduction: the length of a regular expression for a language $L\subseteq {0,1}^n$ is bounded from below by the minimum size of a monotone arithmetic formula computing a polynomial that has $L$ as its set of exponent vectors, viewing words as vectors. This result yields lower bounds for the binomial language of all words with exactly $k$ ones and $n-k$ zeros and for the language of all Dyck words of length $2n$. We also determine the blow-up of language operations (intersection and shuffle) of regular expressions for finite languages. Second, we adapt a lower bound method for multilinear arithmetic formulas by so-called log-product polynomials to regular expressions. With this method we show almost tight lower bounds for the language of all binary numbers with $n$ bits that are divisible by a given odd integer $p$, for the language of all words of length $n$ over a $k$ letter alphabet with an even number of occurrences of each letter and for the language of all permutations of ${1,\dots, n}$.