a(n) = 2^(n+1) - 1 + 3*n.

Terms

a(0) =1a(1) =6a(2) =13a(3) =24a(4) =43a(5) =78a(6) =145a(7) =276a(8) =535a(9) =1050a(10) =2077a(11) =4128a(12) =8227a(13) =16422a(14) =32809a(15) =65580a(16) =131119a(17) =262194a(18) =524341a(19) =1048632a(20) =2097211a(21) =4194366a(22) =8388673a(23) =16777284a(24) =33554503a(25) =67108938a(26) =134217805a(27) =268435536a(28) =536870995a(29) =1073741910