a(n) = Sum_{d|n} (-1)^(n/d-1) * binomial(d+3,4).

Terms

a(0) =1a(1) =4a(2) =16a(3) =29a(4) =71a(5) =115a(6) =211a(7) =289a(8) =511a(9) =649a(10) =1002a(11) =1253a(12) =1821a(13) =2174a(14) =3146a(15) =3505a(16) =4846a(17) =5605a(18) =7316a(19) =8099a(20) =10852a(21) =11653a(22) =14951a(23) =16333a(24) =20546a(25) =21935a(26) =27916a(27) =28904a(28) =35961a(29) =38620