a(n) = 2*4^n + 7.

Terms

a(0) =9a(1) =15a(2) =39a(3) =135a(4) =519a(5) =2055a(6) =8199a(7) =32775a(8) =131079a(9) =524295a(10) =2097159a(11) =8388615a(12) =33554439a(13) =134217735a(14) =536870919a(15) =2147483655a(16) =8589934599a(17) =34359738375a(18) =137438953479a(19) =549755813895