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 A007107 M4668 #58 Jul 04 2023 08:12:38
%S A007107 1,0,0,1,9,216,7570,357435,22040361,1721632024,166261966956,
%T A007107 19459238879565,2714812050902545,445202898702992496,
%U A007107 84798391618743138414,18567039007438379656471,4631381194792101913679985,1305719477625154539392776080,413153055417968797025496881656
%N A007107 Number of labeled 2-regular digraphs with n nodes.
%C A007107 Or number of n X n matrices with exactly two 1's in each row and column which are not in the main diagonal, other entries 0 (cf. A001499). - _Vladimir Shevelev_, Mar 22 2010
%C A007107 Number of 2-factors of the n-crown graph. - _Andrew Howroyd_, Feb 28 2016
%D A007107 R. W. Robinson, personal communication.
%D A007107 R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1982.
%D A007107 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A007107 Alois P. Heinz, <a href="/A007107/b007107.txt">Table of n, a(n) for n = 0..254</a> (first 49 terms from R. W. Robinson)
%H A007107 O. Gonzalez, C. Beltran and I. Santamaria, <a href="http://arxiv.org/abs/1301.6196">On the Number of Interference Alignment Solutions for the K-User MIMO Channel with Constant Coefficients</a>, arXiv preprint arXiv:1301.6196 [cs.IT], 2013. - From _N. J. A. Sloane_, Feb 19 2013
%H A007107 R. J. Mathar, <a href="/A007107/a007107.pdf">OEIS A007107</a>, Mar 15 2019
%F A007107 a(n) = Sum_{k=0..n} Sum_{s=0..k} Sum_{j=0..n-k} (-1)^(k+j-s)*n!*(n-k)!*(2n-k-2j-s)!/(s!*(k-s)!*(n-k-j)!^2*j!*2^(2n-2k-j)). - _Shanzhen Gao_, Nov 05 2007
%F A007107 a(n) ~ 2*sqrt(Pi) * n^(2*n+1/2) / exp(2*n+5/2). - _Vaclav Kotesovec_, May 09 2014
%p A007107 a:= proc(n) option remember; `if`(n<5, ((n-1)*(n-2)/2)^2,
%p A007107 (n-1)*(2*(n^3-2*n^2+n+1)*a(n-1)/(n-2)+((n^2-2*n+2)*
%p A007107 (n+1)*a(n-2) +(2*n^2-6*n+1)*n*a(n-3)+(n-3)*(a(n-4)*
%p A007107 (n^3-5*n^2+3)-(n-4)*(n-1)*(n+1)*a(n-5))))/(2*n))
%p A007107 end:
%p A007107 seq(a(n), n=0..20); # _Alois P. Heinz_, Apr 10 2017
%t A007107 Table[Sum[Sum[Sum[(-1)^(k+j-s)*n!*(n-k)!*(2n-k-2j-s)!/(s!*(k-s)!*(n-k-j)!^2*j!*2^(2n-2k-j)),{j,0,n-k}],{s,0,k}],{k,0,n}],{n,0,20}] (* _Vaclav Kotesovec_, May 09 2014 after _Shanzhen Gao_ *)
%o A007107 (PARI) a(n)=sum(k=0,n, sum(s=0,k, sum(j=0,n-k, (-1)^(k+j-s)*n!*(n-k)!*(2*n-k-2*j-s)!/(s!*(k-s)!*(n-k-j)!^2*j!*2^(2*n-2*k-j))))) \\ _Charles R Greathouse IV_, Feb 08 2017
%Y A007107 Cf. column t=0 of A284989.
%Y A007107 Cf. A007108 (log transform), A197458 (row and column sum <=2), A219889 (unlabeled), A001499 (loops allowed).
%K A007107 nonn,nice
%O A007107 0,5
%A A007107 _N. J. A. Sloane_