a(n) = Sum_{d|n} (-1)^(n/d-1) * binomial(d+2,3).

Terms

a(0) =1a(1) =3a(2) =11a(3) =15a(4) =36a(5) =49a(6) =85a(7) =95a(8) =176a(9) =188a(10) =287a(11) =313a(12) =456a(13) =479a(14) =726a(15) =671a(16) =970a(17) =1024a(18) =1331a(19) =1300a(20) =1866a(21) =1741a(22) =2301a(23) =2265a(24) =2961a(25) =2824a(26) =3830a(27) =3431a(28) =4496a(29) =4514