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 A003425 M4294 #19 May 22 2022 16:21:07 %S A003425 1,1,6,114,5256,507720,93616560,30894489360,17407086641280, %T A003425 16152167106391680,23990233574783750400,55735096448700749203200, %U A003425 198720975339675515386598400,1070118060127292955589511500800,8585695098723146508385537345689600,101432601341702692223559539854263552000 %N A003425 n! times number of posets with n elements. %C A003425 a(n) is the number of nonsingular elements in the semigroup B_n of all binary relations on [n]. A relation A in B_n is nonsingular iff it is regular and row rank(A) = column rank(A) = n. - _Geoffrey Critzer_, May 22 2022 %D A003425 K. K.-H. Butler, A Moore-Penrose inverse for Boolean relation matrices, pp. 18-28 of Combinatorial Mathematics (Proceedings 2nd Australian Conf.), Lect. Notes Math. 403, 1974. %D A003425 K. K.-H. Butler, The Number of Partially Ordered Sets, Journal of Combinatorial Theory (B) 13, 276-289 (1972). %D A003425 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A003425 <a href="/index/Pos#posets">Index entries for sequences related to posets</a> %F A003425 a(n) = A000142(n) * A001035(n). %Y A003425 Cf. A000142, A001035. %K A003425 nonn %O A003425 0,3 %A A003425 _N. J. A. Sloane_