a(n) = 8*4^n - 7.

Terms

a(0) =1a(1) =25a(2) =121a(3) =505a(4) =2041a(5) =8185a(6) =32761a(7) =131065a(8) =524281a(9) =2097145a(10) =8388601a(11) =33554425a(12) =134217721a(13) =536870905a(14) =2147483641a(15) =8589934585a(16) =34359738361a(17) =137438953465a(18) =549755813881