a(n) = 8*(2^n - 1).

Terms

a(0) =8a(1) =24a(2) =56a(3) =120a(4) =248a(5) =504a(6) =1016a(7) =2040a(8) =4088a(9) =8184a(10) =16376a(11) =32760a(12) =65528a(13) =131064a(14) =262136a(15) =524280a(16) =1048568a(17) =2097144a(18) =4194296a(19) =8388600a(20) =16777208a(21) =33554424a(22) =67108856a(23) =134217720a(24) =268435448a(25) =536870904a(26) =1073741816a(27) =2147483640a(28) =4294967288a(29) =8589934584