Number of (binary) heaps of length n whose element set equals {1,2,3,4}.

Terms

a(0) =3a(1) =23a(2) =92a(3) =386a(4) =1004a(5) =3128a(6) =8039a(7) =24725a(8) =56825a(9) =159665a(10) =383447a(11) =1113059a(12) =2366723a(13) =6191435a(14) =14145653a(15) =39253721a(16) =84455981a(17) =223305485a(18) =513899477a(19) =1435302869a(20) =2999680197a(21) =7717870961a(22) =17422727232a(23) =47809866678a(24) =102098766668a(25) =268024219452a(26) =613622726480