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 A386593 #11 Aug 03 2025 17:58:08
%S A386593 1,2,6,30,124
%N A386593 Number of sub-relation-algebras of Re(n), the collection of all binary relations over {1,2,...,n}.
%C A386593 The first four terms are not that difficult to verify. In fact, for n < 5 the subalgebra lattice of Re(n) is the dual of the subgroup lattice of S_n. Hence a(n) = A005432(n) for n < 5.
%C A386593 Relation algebras are closed under union, intersection, complement, composition, inverse, and identity.
%D A386593 Bjarni Jónsson, Maximal algebras of binary relations. In: Contributions to Group Theory, Contemporary Mathematics, vol. 33, pp. 299-307. Amer. Math. Soc., Providence (1984).
%Y A386593 Cf. A005432.
%K A386593 nonn,hard,more
%O A386593 1,2
%A A386593 _Jeremy F. Alm_, Jul 26 2025