a(n) = n^2 * (n + 1)/2 - Sum_{k=1..n} sigma_2(k).

Terms

a(0) =0a(1) =0a(2) =2a(3) =3a(4) =12a(5) =13a(6) =33a(7) =40a(8) =66a(9) =81a(10) =135a(11) =135a(12) =212a(13) =249a(14) =319a(15) =354a(16) =489a(17) =511a(18) =681a(19) =725a(20) =876a(21) =981a(22) =1233a(23) =1235a(24) =1509a(25) =1660a(26) =1920a(27) =2032a(28) =2437a(29) =2472