a(n) = T(n, 4) with T(n, k) = Sum_{d|k} phi(d)*binomial(n - 1 + k/d, k/d).

Terms

a(0) =0a(1) =4a(2) =12a(3) =27a(4) =53a(5) =95a(6) =159a(7) =252a(8) =382a(9) =558a(10) =790a(11) =1089a(12) =1467a(13) =1937a(14) =2513a(15) =3210a(16) =4044a(17) =5032a(18) =6192a(19) =7543a(20) =9105a(21) =10899a(22) =12947a(23) =15272a(24) =17898a(25) =20850a(26) =24154a(27) =27837a(28) =31927a(29) =36453