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 A052791 #33 Sep 02 2022 17:54:01
%S A052791 0,0,0,6,72,540,3240,17010,81648,367416,1574640,6495390,25981560,
%T A052791 101328084,386889048,1450833930,5356925280,19514513520,70252248672,
%U A052791 250273635894,883318714920,3091615502220,10739295955080,37050571045026,127030529297232,433058622604200,1468633589701200
%N A052791 3^(n-3)*n*(n-1)*(n-2).
%C A052791 The number of surjective functions f:{1,2,...,n}->{1,2,3} with a designated pre-image of 1, 2, and 3.
%H A052791 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=748">Encyclopedia of Combinatorial Structures 748</a>
%H A052791 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (12,-54,108,-81).
%F A052791 E.g.f.: x^3*exp(x)^3
%F A052791 Recurrence: {a(1)=0, a(2)=0, a(3)=6, (-3*n-3)*a(n)+(-2+n)*a(n+1)}.
%F A052791 a(n) = n!*sum(i+j+k=n, ijk/(i!j!k!)) - _Benoit Cloitre_, Nov 11 2004
%F A052791 G.f.: 6*x^3 / (3*x-1)^4. - _Colin Barker_, Jun 04 2013
%p A052791 spec := [S,{B=Set(Z),S=Prod(Z,Z,Z,B,B,B)},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t A052791 Range[0, 20]! CoefficientList[Series[(x Exp[x])^3, {x, 0, 20}], x]
%t A052791 LinearRecurrence[{12,-54,108,-81},{0,0,0,6},30] (* _Harvey P. Dale_, Sep 02 2022 *)
%o A052791 (PARI) a(n)=3^(n-3)*n*(n-1)*(n-2); /* _Joerg Arndt_, Sep 16 2012 */
%Y A052791 Cf. A001815.
%K A052791 easy,nonn
%O A052791 0,4
%A A052791 encyclopedia(AT)pommard.inria.fr, Jan 25 2000
%E A052791 Edited by _N. J. A. Sloane_, Dec 24 2010