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

Terms

a(0) =2a(1) =11a(2) =47a(3) =191a(4) =767a(5) =3071a(6) =12287a(7) =49151a(8) =196607a(9) =786431a(10) =3145727a(11) =12582911a(12) =50331647a(13) =201326591a(14) =805306367a(15) =3221225471a(16) =12884901887a(17) =51539607551a(18) =206158430207a(19) =824633720831