a(n) = Sum_{k=3..n} binomial(k,3) * floor(n/k).

Terms

a(0) =0a(1) =0a(2) =1a(3) =5a(4) =15a(5) =36a(6) =71a(7) =131a(8) =216a(9) =346a(10) =511a(11) =756a(12) =1042a(13) =1441a(14) =1907a(15) =2527a(16) =3207a(17) =4128a(18) =5097a(19) =6371a(20) =7737a(21) =9442a(22) =11213a(23) =13538a(24) =15848a(25) =18734a(26) =21744a(27) =25423a(28) =29077a(29) =33743