a(n) = 13*2^n-1.

Terms

a(0) =12a(1) =25a(2) =51a(3) =103a(4) =207a(5) =415a(6) =831a(7) =1663a(8) =3327a(9) =6655a(10) =13311a(11) =26623a(12) =53247a(13) =106495a(14) =212991a(15) =425983a(16) =851967a(17) =1703935a(18) =3407871a(19) =6815743a(20) =13631487a(21) =27262975a(22) =54525951a(23) =109051903a(24) =218103807a(25) =436207615a(26) =872415231a(27) =1744830463a(28) =3489660927a(29) =6979321855