Four times the sum of all divisors of all positive integers <= n.

Terms

a(0) =4a(1) =16a(2) =32a(3) =60a(4) =84a(5) =132a(6) =164a(7) =224a(8) =276a(9) =348a(10) =396a(11) =508a(12) =564a(13) =660a(14) =756a(15) =880a(16) =952a(17) =1108a(18) =1188a(19) =1356a(20) =1484a(21) =1628a(22) =1724a(23) =1964a(24) =2088a(25) =2256a(26) =2416a(27) =2640a(28) =2760a(29) =3048