a(n) = 4^n - 2^n + 1.

Terms

a(0) =1a(1) =3a(2) =13a(3) =57a(4) =241a(5) =993a(6) =4033a(7) =16257a(8) =65281a(9) =261633a(10) =1047553a(11) =4192257a(12) =16773121a(13) =67100673a(14) =268419073a(15) =1073709057a(16) =4294901761a(17) =17179738113a(18) =68719214593a(19) =274877382657