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 A081131 #31 May 21 2025 20:30:37
%S A081131 0,0,1,9,96,1250,19440,352947,7340032,172186884,4500000000,
%T A081131 129687123005,4086546038784,139788510734886,5159146026151936,
%U A081131 204350482177734375,8646911284551352320,389289535005334947848,18580248257778920521728,937146152681201173795569
%N A081131 a(n) = n^(n-2) * binomial(n,2).
%C A081131 Main diagonal of A081130.
%C A081131 a(n) is the number of partial functions f: {1,2,...,n} -> {1,2,...,n} that have exactly 2 undefined elements. - _Geoffrey Critzer_, Feb 08 2012
%C A081131 a(n+1) is the determinant of the circulant matrix having (n-1, n-2, ..., 0) as first row, for n >= 1. See A070896 for a variant, and A303260 for a related sequence. - _M. F. Hasler_, Apr 23 2018
%C A081131 a(n) is the number of birooted labeled trees on n nodes. - _Brendan McKay_, May 01 2018
%H A081131 Vincenzo Librandi, <a href="/A081131/b081131.txt">Table of n, a(n) for n = 0..200</a>
%F A081131 a(0) = a(1) = 0, a(n) = n^(n-2)*binomial(n,2).
%F A081131 E.g.f.: T(x)^2/(2*(1-T(x))) where T(x) is the e.g.f. for A000169. - _Geoffrey Critzer_, Feb 08 2012
%t A081131 Join[{0},Table[n^(n-2) Binomial[n, 2], {n, 1, 20}]] (* _Vladimir Joseph Stephan Orlovsky_, Apr 19 2011 *)
%o A081131 (Magma) [n lt 2 select 0 else n^(n-2)*Binomial(n,2): n in [0..20]]; // _G. C. Greubel_, May 18 2021
%o A081131 (Sage) [0 if (n<2) else n^(n-2)*binomial(n,2) for n in (0..20)] # _G. C. Greubel_, May 18 2021
%Y A081131 Cf. A070896, A081129, A081130, A303260.
%Y A081131 Sequences of the form (n+m)^n*binomial(n+2,2): A081133 (m=0), A081132 (m=1), this sequence (m=2), A053507 (m=3), A081196 (m=4).
%K A081131 easy,nonn
%O A081131 0,4
%A A081131 _Paul Barry_, Mar 08 2003