a(n) = 2*4^n + 3.

Terms

a(0) =5a(1) =11a(2) =35a(3) =131a(4) =515a(5) =2051a(6) =8195a(7) =32771a(8) =131075a(9) =524291a(10) =2097155a(11) =8388611a(12) =33554435a(13) =134217731a(14) =536870915a(15) =2147483651a(16) =8589934595a(17) =34359738371a(18) =137438953475a(19) =549755813891