a(n) = Sum_{k=1..n} binomial(k+2,2) * floor(n/k).

Terms

a(0) =3a(1) =12a(2) =25a(3) =49a(4) =73a(5) =120a(6) =159a(7) =228a(8) =296a(9) =392a(10) =473a(11) =626a(12) =734a(13) =899a(14) =1069a(15) =1291a(16) =1465a(17) =1757a(18) =1970a(19) =2312a(20) =2614a(21) =2977a(22) =3280a(23) =3803a(24) =4178a(25) =4670a(26) =5144a(27) =5759a(28) =6227a(29) =6993