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 A118100 #16 Jun 01 2025 17:16:24
%S A118100 1,2,5,17,75,400,2543,19834,241639,11787482,3530717819
%N A118100 Number of commutative semigroups of order <= n.
%C A118100 A001426(n) is the number of commutative semigroups of order n. A001426(n) + A079193(n) + A079196(n) + A079199(n) = A001329(n). 2, 5, 17, 2543 and 241639 are primes.
%H A118100 <a href="/index/Se#semigroups">Index entries for sequences related to semigroups</a>.
%F A118100 a(n) = Sum_{i=1..n} A001426(i).
%e A118100 a(8) = 1 + 1 + 3 + 12 + 58 + 325 + 2143 + 17291 + 221805 = 241639.
%Y A118100 Cf. A001423, A001426, A023815, A027851, A058105, A058116.
%K A118100 nonn,more
%O A118100 0,2
%A A118100 _Jonathan Vos Post_, May 11 2006
%E A118100 a(9)-a(10) added using the terms in A001426 by _Miles Englezou_, May 27 2025