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 A055533 #17 Mar 16 2021 08:31:09 %S A055533 1,12,108,960,9000,90720,987840,11612160,146966400,1995840000, %T A055533 28979596800,448345497600,7366565606400,128152088064000, %U A055533 2353813862400000,45527990796288000,925143000477696000 %N A055533 Number of labeled order relations on n nodes in which longest chain has n-1 nodes. %C A055533 Also number of labeled acyclic digraphs with n nodes and binomial(n,2)-1 arcs, cf. A081064. - _Vladeta Jovovic_, Jul 14 2008 %D A055533 L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 60. %F A055533 E.g.f.: (x^2/2)(1+x)/(1-x)^3. a(n)=n!(n-1)^2/2, if n>1. - _Vladeta Jovovic_, Dec 01 2002 %F A055533 D-finite with recurrence (n-2)^2*a(n) -n*(n-1)^2*a(n-1)=0. - _R. J. Mathar_, Mar 08 2021 %o A055533 (PARI) a(n)=if(n<2,0,n!/2*(n-1)^2) /* _Michael Somos_, Mar 06 2004 */ %Y A055533 A column or diagonal of triangle in A342587. %K A055533 nonn %O A055533 2,2 %A A055533 _N. J. A. Sloane_, Jul 10 2000 %E A055533 More terms from _Vladeta Jovovic_, Dec 01 2002