a(n) = Sum_{k=1..n} (-1)^(k+1) * floor((n/k)^3).

Terms

a(0) =1a(1) =7a(2) =25a(3) =57a(4) =114a(5) =194a(6) =310a(7) =460a(8) =658a(9) =902a(10) =1200a(11) =1557a(12) =1981a(13) =2474a(14) =3045a(15) =3690a(16) =4431a(17) =5257a(18) =6186a(19) =7212a(20) =8350a(21) =9601a(22) =10969a(23) =12461a(24) =14088a(25) =15844a(26) =17746a(27) =19791a(28) =21990a(29) =24343