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

Terms

a(0) =-1a(1) =7a(2) =47a(3) =223a(4) =959a(5) =3967a(6) =16127a(7) =65023a(8) =261119a(9) =1046527a(10) =4190207a(11) =16769023a(12) =67092479a(13) =268402687a(14) =1073676287a(15) =4294836223a(16) =17179607039a(17) =68718952447a(18) =274876858367