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 A019266 #40 Jul 27 2022 03:46:40
%S A019266 1,0,4,12,56,280,1488,8232,47008,274824,1636520,9890584,60510480,
%T A019266 374019776,2332131872,14651535840,92653845120,589317728376,
%U A019266 3767523372432,24196006128592,156030800105840,1009911004804296,6558631830442384,42723991459518368,279091277437885920
%N A019266 Cycle class sequence c(2n) (the number of true cycles of length 2n in which a certain node is included) for square lattice.
%H A019266 Alois P. Heinz, <a href="/A019266/b019266.txt">Table of n, a(n) for n = 0..65</a>
%H A019266 G. Thimm, <a href="/thimm1.html">Cycle sequences of crystal structures</a>
%H A019266 G. Thimm and W. E. Klee, <a href="https://doi.org/10.1016/S0144-2449(97)00122-X">Zeolite cycle sequences</a>, Zeolites, 19, pp. 422-424, 1997.
%F A019266 a(n) = A010566(n) / 2 = 2 * n * A002931(n) for n > 0. - _Andrey Zabolotskiy_, Jul 26 2022
%t A019266 (Length@FindCycle[{NearestNeighborGraph[Tuples[Range[2 # + 4], 2], {All, 1.}], {#+2,#+2}}, {2 #}, All]) & /@ Range[11] (* _Gabriel B. Apolinario_, Jan 07 2017 *)
%Y A019266 Cf. A002931, A010566.
%K A019266 nonn
%O A019266 0,3
%A A019266 Georg Thimm (mgeorg(AT)ntu.edu.sg)
%E A019266 a(11) from _Gabriel B. Apolinario_, Jan 07 2017
%E A019266 a(12)-a(18) from _Aleksandr D. Krotov_, Jan 07 2018
%E A019266 a(19)-a(20) from _Aleksandr D. Krotov_, Mar 22 2019
%E A019266 Terms a(21) and beyond added by _Andrey Zabolotskiy_, Jul 26 2022, using A002931