Author

Vincent Vatter

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Preprint Oct 2026

Deciding lettericity is NP-complete, even for four-colorable comparability graphs

We prove that deciding whether a graph has lettericity at most k is NP-complete, even for four-colorable comparability graphs. Our reduction maps a bipartite graph G to the graph obtained from its incidence graph by inflating each vertex by a clique or an independent set of three vertices. The lettericity of this graph...

Vincent Vatter · 1 citation · ⚡1

Preprint Aug 2026

Log-concavity of subsequence counts of words

Given a word over a finite alphabet, consider the sequence that counts, for each length, the number of distinct subsequences of that length. In 1976, Chase proved that this sequence is log-concave. His proof uses a triangular array indexed by the prefixes of the word together with a meticulous analysis of ratios of sev...

Vincent Vatter · 0 citations