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 A001866 M5170 N2245 #29 Dec 27 2024 03:49:24
%S A001866 0,0,1,24,936,56640,4968000,598328640,94916183040,19200422062080,
%T A001866 4826695329792000,1476585999504000000,540272647694971699200,
%U A001866 233019960215154829516800,117009251702203840384204800,67680314823703303654732800000,44677678066673631080900198400000
%N A001866 Number of connected graphs with n nodes and n edges.
%C A001866 Or number of n X n (0,1) matrices with two 1's in each row the permanent of which equals to 2. Note that, if (0,1) matrix with two 1's in each row has positive permanent, then it is equal to a power of 2. - _Vladimir Shevelev_, Mar 25 2010
%D A001866 V. S. Shevelev, On the permanent of the stochastic (0,1)-matrices with equal row sums, Izvestia Vuzov of the North-Caucasus region, Nature sciences 1 (1997), 21-38 (in Russian). - _Vladimir Shevelev_, Mar 25 2010
%D A001866 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D A001866 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A001866 T. D. Noe, <a href="/A001866/b001866.txt">Table of n, a(n) for n = 0..100</a>
%H A001866 T. L. Austin, R. E. Fagen, W. F. Penney, and John Riordan, <a href="http://projecteuclid.org/euclid.aoms/1177706204">The number of components in random linear graphs</a>, Ann. Math. Statist 30 1959 747-754.
%H A001866 J. Riordan, <a href="/A001863/a001863.pdf">Letter to N. J. A. Sloane, Aug. 1970</a>
%F A001866 Explicit formula: a(n) = (n!^2*n^(n-1)/2)*Sum_{k=2..n} n^(-k)/(n-k)!; Recursion: a(2)=1, for n>=3, a(n) = n!*((n-1)!/2+Sum_{k=2..n-1} (-1)^(n+k+1)*k^(n-k)*binomial(n,k)*a(k)/k!). - _Vladimir Shevelev_, Mar 25 2010
%F A001866 a(n) ~ Pi * n^(2*n) / (2*exp(n)). - _Vaclav Kotesovec_, Nov 30 2017
%t A001866 Join[{0}, Table[(n!^2*n^(n - 1)/2)*Sum[n^(-k)/(n - k)!, {k, 2, n}], {n, 20}]] (* _T. D. Noe_, Aug 10 2012 *)
%Y A001866 Cf. A174586.
%K A001866 nonn
%O A001866 0,4
%A A001866 _N. J. A. Sloane_