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 A281583 #17 Feb 14 2021 08:31:49
%S A281583 140450,16091936,2415542018,400448833106,69206906601800,
%T A281583 12190695635108354,2167175327735637122,387018647188487143424,
%U A281583 69272289588070930561250,12413316310203106546620386,2225719417041514241075539592,399192630631160441128470998546
%N A281583 Number of perfect matchings in the graph C_9 X C_{2n}.
%C A281583 For odd values of m the order of recurrence relation for the number of perfect matchings in C_{m} X C_{2n} graph does not exceed 3^floor(m/2).
%H A281583 Seiichi Manyama, <a href="/A281583/b281583.txt">Table of n, a(n) for n = 2..443</a>
%H A281583 S. N. Perepechko, <a href="http://www.jip.ru/2016/333-361-2016.pdf">The number of perfect matchings on C_m X C_n graphs</a>, (in Russian), Information Processes, 2016, V.16, No.4, pp.333-361.
%H A281583 Sergey Perepechko, <a href="/A281583/a281583.pdf">Generating function</a>, in Maple notation.
%F A281583 a(n) = sqrt( Product_{j=1..n} Product_{k=1..9} (4*sin((2*j-1)*Pi/(2*n))^2 + 4*sin((2*k-1)*Pi/9)^2) ). - _Seiichi Manyama_, Feb 14 2021
%o A281583 (PARI) default(realprecision, 120);
%o A281583 a(n) = round(sqrt(prod(j=1, n, prod(k=1, 9, 4*sin((2*j-1)*Pi/(2*n))^2+4*sin((2*k-1)*Pi/9)^2)))); \\ _Seiichi Manyama_, Feb 14 2021
%Y A281583 Column k=9 of A341533.
%Y A281583 Cf. A253678, A230033, A232804, A231485, A220864, A231087.
%K A281583 nonn
%O A281583 2,1
%A A281583 _Sergey Perepechko_, Jan 25 2017