Codes with restricted overlaps: expandability, constructions, and bounds

Abstract: Consider a $q$-ary block code satisfying the property that no $l$-letters long codeword's prefix occurs as a suffix of any codeword for $l$ inside some interval. We determine a general upper bound on the maximum size of these codes and a tighter bound for codes where overlaps with lengths not exceeding $k$ are prohibited. We then provide constructions for codes with various restrictions on overlap lengths and use them to determine lower bounds on the maximum sizes. In particular, we construct $(1,k)$-overlap-free codes where $k \geq n/2$ and $n$ denotes the block size, expand a known construction of $(k,n-1)$-overlap-free codes, and combine the ideas behind both constructions to obtain $(t_1,t_2)$-overlap-free codes and codes that are simultaneously $(1,k)$- and $(n-k,n-1)$-overlap-free for some $k < n/2$. In the case when overlaps of lengths between 1 and $k$ are prohibited, we complete the characterisation of non-expandable codes started by Cai, Wang, and Feng (2023).