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arXiv · 0906.2183

Enumeration of non-crossing pairings on bit strings

Todd Kemp·Karl Mahlburg·Amarpreet Rattan·Clifford Smyth
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Abstract

A non-crossing pairing on a bitstring matches 1s and 0s in a manner such that the pairing diagram is nonintersecting. By considering such pairings on arbitrary bitstrings $1^{n_1} 0^{m_1} ... 1^{n_r} 0^{m_r}$, we generalize classical problems from the theory of Catalan structures. In particular, it is very difficult to find useful explicit formulas for the enumeration function $ϕ(n_1, m_1, ..., n_r, m_r)$, which counts the number of pairings as a function of the underlying bitstring. We determine explicit formulas for $ϕ$, and also prove general upper bounds in terms of Fuss-Catalan numbers by relating non-crossing pairings to other generalized Catalan structures (that are in some sense more natural). This enumeration problem arises in the theory of random matrices and free probability.

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Todd Kemp, Karl Mahlburg, Amarpreet Rattan, Clifford Smyth. 2009-06-17. Enumeration of non-crossing pairings on bit strings. https://arxiv.org/abs/0906.2183

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