Proving some conjectures on Kekulé numbers for certain benzenoids by using Chebyshev polynomials

Abstract

In chemistry, Cyvin-Gutman enumerates Kekulé 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)+∑k=0⌊n−12⌋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, we give explicit formula of the column generating function, which is also the generating function F(n,x) studied by Bóna, 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.