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

Terms

a(0) =1a(1) =1a(2) =4a(3) =15a(4) =46a(5) =125a(6) =316a(7) =763a(8) =1786a(9) =4089a(10) =9208a(11) =20471a(12) =45046a(13) =98293a(14) =212980a(15) =458739a(16) =983026a(17) =2097137a(18) =4456432a(19) =9437167a(20) =19922926a(21) =41943021a(22) =88080364a(23) =184549355a(24) =385875946a(25) =805306345a(26) =1677721576a(27) =3489660903a(28) =7247757286a(29) =15032385509