Number of set partitions of [n] with symmetric block size list of length three.

Terms

a(0) =1a(1) =2a(2) =7a(3) =19a(4) =56a(5) =160a(6) =463a(7) =1337a(8) =3874a(9) =11241a(10) =32682a(11) =95172a(12) =277577a(13) =810706a(14) =2370839a(15) =6941473a(16) =20345618a(17) =59692831a(18) =175295996a(19) =515217034a(20) =1515478535a(21) =4460940067a(22) =13140081770a(23) =38729776774a(24) =114221851951a(25) =337050020750a(26) =995097461503