Catafusenes (see reference for precise definition).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =2a(6) =16a(7) =80a(8) =473a(9) =2517a(10) =13431a(11) =69357a(12) =353943a(13) =1780894a(14) =8886254a(15) =44024308a(16) =217021967a(17) =1065653519a(18) =5217704239a(19) =25491204743a(20) =124335254416a(21) =605720277647