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

Terms

a(0) =1a(1) =4a(2) =15a(3) =30a(4) =66a(5) =115a(6) =200a(7) =295a(8) =471a(9) =659a(10) =946a(11) =1259a(12) =1715a(13) =2194a(14) =2920a(15) =3591a(16) =4561a(17) =5585a(18) =6916a(19) =8216a(20) =10082a(21) =11823a(22) =14124a(23) =16389a(24) =19350a(25) =22174a(26) =26004a(27) =29435a(28) =33931a(29) =38445