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 A381089 #16 Feb 21 2025 12:29:49
%S A381089 1,0,7,86,2846,285984,96348100,112089342912,458072631172864,
%T A381089 6665705090236713408,349377212708652631367712,
%U A381089 66602723210653815331014240512,46557323276092409455163109412993536,120168498152266645852126063743794842575872
%N A381089 Number of binary relations on n unlabeled points without isolated points.
%C A381089 Equivalently, the number of simple digraphs on n unlabeled two-colored nodes where each node is connected to at least one other node.
%H A381089 Alois P. Heinz, <a href="/A381089/b381089.txt">Table of n, a(n) for n = 0..50</a>
%F A381089 a(n) = A000595(n) - Sum_{i=1..n} (i+1)*a(n-i).
%e A381089 For n = 2 there are 10 (=A000595(2)) - 3 (=number of relations with isolated points) = 7 = a(2) relations.
%e A381089 For n = 3 there are 104 (=A000595(3)) - 2 * 7 (=number of relations with exactly one isolated point) - 3 * 0 (=number of relations with exactly two isolated points) - 4 * 1 (=number of relations with exactly three isolated points) = 86 = a(3) relations.
%Y A381089 Cf. A000595.
%K A381089 nonn
%O A381089 0,3
%A A381089 _Peter Dolland_, Feb 13 2025