a(n) = Sum_{d|n} mu(n/d) * binomial(d,3).

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =10a(5) =19a(6) =35a(7) =52a(8) =83a(9) =110a(10) =165a(11) =196a(12) =286a(13) =329a(14) =444a(15) =504a(16) =680a(17) =713a(18) =969a(19) =1016a(20) =1294a(21) =1375a(22) =1771a(23) =1752a(24) =2290a(25) =2314a(26) =2841a(27) =2908a(28) =3654a(29) =3476