a(1) = 2, a(2) = 4; a(n) = 2*a(n-1) - 1.

Terms

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