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

Terms

a(0) =1a(1) =7a(2) =27a(3) =56a(4) =118a(5) =196a(6) =324a(7) =448a(8) =685a(9) =901a(10) =1233a(11) =1549a(12) =2019a(13) =2445a(14) =3157a(15) =3664a(16) =4482a(17) =5262a(18) =6290a(19) =7128a(20) =8536a(21) =9598a(22) =11118a(23) =12392a(24) =14255a(25) =15743a(26) =18087a(27) =19711a(28) =22149a(29) =24417