Number of unlabeled 4-trees on n vertices.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =1a(5) =1a(6) =2a(7) =5a(8) =15a(9) =64a(10) =331a(11) =2150a(12) =15817a(13) =127194a(14) =1077639a(15) =9466983a(16) =85252938a(17) =782238933a(18) =7283470324a(19) =68639621442a(20) =653492361220