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 A361920 #20 Oct 30 2023 15:33:22 %S A361920 1,1,2,5,16,61,280,1501,9394,68647,591570,6108298,77162708,1219779207, %T A361920 24648006828,647865966973,22437052221282,1032905858402302, %U A361920 63591727342096158,5258562027225785955,586001891321599337103,88241281449605821921186,17996565026907866304071630 %N A361920 Number of unlabeled ranked posets with n elements. %C A361920 A partially ordered set is ranked if there is a function from the poset elements to the integers such that the function value of a covering element is precisely one larger than the function value of the covered element. This is called graded by some authors. %H A361920 Andrew Howroyd, <a href="/A361920/b361920.txt">Table of n, a(n) for n = 0..40</a> %e A361920 For n=5, A000112(n) - a(n) = 63 - 61 = 2 because we have 2 posets with 5 elements that are not ranked: a<b<c<d a<e<d and a<c<e a<d b<d b<e where < means "is covered by". - _Geoffrey Critzer_, Oct 29 2023 %o A361920 (Sage) sum(1 for P in posets(n) if P.is_ranked()) %o A361920 (PARI) \\ See PARI link in A361953 for program code. %o A361920 A361920seq(20) \\ _Andrew Howroyd_, Apr 01 2023 %Y A361920 Row sums of A361953. %Y A361920 Cf. A000112, A001833 (labeled), A223911, A361912, A361955. %K A361920 nonn %O A361920 0,3 %A A361920 _Martin Rubey_, Mar 29 2023 %E A361920 Terms a(8) and beyond from _Andrew Howroyd_, Mar 31 2023