This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A309117 #12 Jul 14 2019 07:55:43 %S A309117 1,1,5,15,56,203,749,2777,10293,38240,141997,527593,1960029,7282483, %T A309117 27057400,100531559,373522965,1387822193,5156442953,19158736256, %U A309117 71184183353,264484479633,982690786037,3651182836279,13565952140920,50404229548515,187276671274621 %N A309117 Number of perfect matchings on a triangular lattice of width 4 and length n. %H A309117 S. N. Perepechko, <a href="http://dx.doi.org/10.13140/RG.2.2.20780.82568">Number of perfect matchings on triangular lattices of fixed width</a>, DIMA'2015 slides. %F A309117 G.f.: (1-z)*(1+z)*(1-z-5*z^2-z^3+z^4)/((1+z-3*z^2-3*z^3+z^4)*(1-3*z-3*z^2+z^3+z^4)). %Y A309117 Cf. A011900, A307349. %K A309117 nonn,easy %O A309117 0,3 %A A309117 _Sergey Perepechko_, Jul 13 2019