a(n) = 3*4^n + 1.

Terms

a(0) =4a(1) =13a(2) =49a(3) =193a(4) =769a(5) =3073a(6) =12289a(7) =49153a(8) =196609a(9) =786433a(10) =3145729a(11) =12582913a(12) =50331649a(13) =201326593a(14) =805306369a(15) =3221225473a(16) =12884901889a(17) =51539607553a(18) =206158430209a(19) =824633720833