a(n) = 3*2^n + 1.

Terms

a(0) =4a(1) =7a(2) =13a(3) =25a(4) =49a(5) =97a(6) =193a(7) =385a(8) =769a(9) =1537a(10) =3073a(11) =6145a(12) =12289a(13) =24577a(14) =49153a(15) =98305a(16) =196609a(17) =393217a(18) =786433a(19) =1572865a(20) =3145729a(21) =6291457a(22) =12582913a(23) =25165825a(24) =50331649a(25) =100663297a(26) =201326593a(27) =402653185a(28) =805306369a(29) =1610612737