a(n) = C(n+2,3) + C(n,3) + C(n-1,3).

Terms

a(0) =1a(1) =4a(2) =11a(3) =25a(4) =49a(5) =86a(6) =139a(7) =211a(8) =305a(9) =424a(10) =571a(11) =749a(12) =961a(13) =1210a(14) =1499a(15) =1831a(16) =2209a(17) =2636a(18) =3115a(19) =3649a(20) =4241a(21) =4894a(22) =5611a(23) =6395a(24) =7249a(25) =8176a(26) =9179a(27) =10261a(28) =11425a(29) =12674