Basis for code in A075928.

Terms

a(0) =15a(1) =51a(2) =85a(3) =150a(4) =771a(5) =1285a(6) =2310a(7) =4369a(8) =8466a(9) =16660a(10) =33047a(11) =196611a(12) =327685a(13) =589830a(14) =1114129a(15) =2162706a(16) =4259860a(17) =8454167a(18) =16843009a(19) =33620226a(20) =67174660a(21) =134283527a(22) =268501264a(23) =536936723a(24) =1073807637a(25) =2147549462a(26) =12884901891