Let r(n) be the real positive root of Sum_{k=1..n} x^k = 1, then a(n) = round(1/(r(n) - 1/2)).

Terms

a(0) =2a(1) =8a(2) =23a(3) =53a(4) =115a(5) =242a(6) =496a(7) =1006a(8) =2028a(9) =4074a(10) =8168a(11) =16358a(12) =32740a(13) =65506a(14) =131040a(15) =262110a(16) =524252a(17) =1048538a(18) =2097112a(19) =4194262a(20) =8388564a(21) =16777170a(22) =33554384a(23) =67108814a(24) =134217676a(25) =268435402a(26) =536870856a(27) =1073741766a(28) =2147483588