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

Terms

a(0) =3a(1) =4a(2) =7a(3) =14a(4) =29a(5) =60a(6) =123a(7) =250a(8) =505a(9) =1016a(10) =2039a(11) =4086a(12) =8181a(13) =16372a(14) =32755a(15) =65522a(16) =131057a(17) =262128a(18) =524271a(19) =1048558a(20) =2097133a(21) =4194284a(22) =8388587a(23) =16777194a(24) =33554409a(25) =67108840a(26) =134217703a(27) =268435430a(28) =536870885a(29) =1073741796