Bukh–Borisov deletion-distance conjecture for binary words

For a binary word WW, let g(W)g(W) be the minimum number of letters that must be deleted from WW to obtain a shuffle square, and let g(n)g(n) be the maximum of g(W)g(W) over binary words of length nn. A shuffle square is a word containing two disjoint equal subsequences whose total length is the word's length.

Bukh–Borisov deletion-distance conjecture. There is a constant cc such that, for every nn,

g(n)cn,g(n)\geqslant c\sqrt n,

that is, there exists a binary word of length nn with largest twins leaving out at least cnc\sqrt n elements.

Known bounds include g(n)=o(n)g(n)=o(n) and g(n)=Ω(n1/3)g(n)=\Omega(n^{1/3}). The conjecture predicts the stronger lower bound of order n\sqrt n.

Sources & referencesView supporting material

Primary source

Jarosław Grytczuk, Bartłomiej Pawlik and Andrzej Ruciński, “Shuffle squares and ordered nest-free graphs”, arXiv:2503.22043 (2025).

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