Proving some conjectures on Kekul\'{e} numbers for certain benzenoids by using Chebyshev polynomials

TL;DR

This paper proves conjectures related to Kekulé numbers for certain benzenoids by deriving explicit formulas for their generating functions using Chebyshev polynomials, connecting chemistry, graph theory, and combinatorics.

Contribution

It provides explicit formulas for the generating functions of the Kekulé numbers, confirming several conjectures through Chebyshev polynomial techniques.

Findings

Explicit formula for the column generating function derived.

Trig function representations obtained via Chebyshev polynomials.

All conjectures on the generating functions proved.

Abstract

In chemistry, Cyvin-Gutman enumerates Kekul\'{e} numbers for certain benzenoids and record it as $A050446$ on OEIS. This number is exactly the two variable array $T(n,m)$ defined by the recursion $T(n, m) = T(n, m-1) + \sum^{\lfloor\frac{n-1}{2}\rfloor}_{k=0} T(2k, m-1)T(n-1-2k, m)$, where $T(n,0)=T(0,m)=1$ for all nonnegative integers $m,n$. Interestingly, this number also appeared in the context of weighted graphs, graph polytopes, magic labellings, and unit primitive matrices, studied by different authors. Several interesting conjectures were made on the OEIS. These conjectures are related to both the row and column generating function of $T(n,m)$. In this paper, give explicit formula of the column generating function, which is also the generating function $F(n,x)$ studied by B\'{o}na, Ju, and Yoshida. We also get trig function representations by using Chebyshev polynomials of the second kind. This allows us to prove all these conjectures.