a(n) = A255302(2^n - 1).

Terms

a(0) =1a(1) =4a(2) =16a(3) =60a(4) =216a(5) =768a(6) =2728a(7) =9704a(8) =34552a(9) =123064a(10) =438328a(11) =1561176a(12) =5560248a(13) =19803096a(14) =70529656a(15) =251194904a(16) =894643768a(17) =3186321112a(18) =11348251384a(19) =40417397400a(20) =143948695992a(21) =512680882776