Sum of all divisors of all positive integers <= 2^n.

Terms

a(0) =1a(1) =4a(2) =15a(3) =56a(4) =220a(5) =857a(6) =3403a(7) =13535a(8) =54077a(9) =215900a(10) =862954a(11) =3450545a(12) =13802279a(13) =55201838a(14) =220792018a(15) =883134861a(16) =3532518195a(17) =14129951284a(18) =56519699688a(19) =226078355122a(20) =904312961284