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

Terms

a(0) =3a(1) =6a(2) =13a(3) =28a(4) =59a(5) =122a(6) =249a(7) =504a(8) =1015a(9) =2038a(10) =4085a(11) =8180a(12) =16371a(13) =32754a(14) =65521a(15) =131056a(16) =262127a(17) =524270a(18) =1048557a(19) =2097132a(20) =4194283a(21) =8388586a(22) =16777193a(23) =33554408a(24) =67108839a(25) =134217702a(26) =268435429a(27) =536870884a(28) =1073741795a(29) =2147483618