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

Terms

a(0) =3a(1) =9a(2) =33a(3) =129a(4) =513a(5) =2049a(6) =8193a(7) =32769a(8) =131073a(9) =524289a(10) =2097153a(11) =8388609a(12) =33554433a(13) =134217729a(14) =536870913a(15) =2147483649a(16) =8589934593a(17) =34359738369a(18) =137438953473a(19) =549755813889