a(n) = Sum_{k=1..n} sigma_2(k) * floor(n/k).

Terms

a(0) =1a(1) =7a(2) =18a(3) =45a(4) =72a(5) =138a(6) =189a(7) =301a(8) =403a(9) =565a(10) =688a(11) =985a(12) =1156a(13) =1462a(14) =1759a(15) =2212a(16) =2503a(17) =3115a(18) =3478a(19) =4207a(20) =4768a(21) =5506a(22) =6037a(23) =7269a(24) =7947a(25) =8973a(26) =9895a(27) =11272a(28) =12115a(29) =13897