Number of (binary) heaps on n elements.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =8a(6) =20a(7) =80a(8) =210a(9) =896a(10) =3360a(11) =19200a(12) =79200a(13) =506880a(14) =2745600a(15) =21964800a(16) =108108000a(17) =820019200a(18) =5227622400a(19) =48881664000a(20) =319258368000