a(n) = 1 - (7/6)*n + (2/3)*n^3 + (1/2)*n^4.

Terms

a(0) =1a(1) =1a(2) =12a(3) =56a(4) =167a(5) =391a(6) =786a(7) =1422a(8) =2381a(9) =3757a(10) =5656a(11) =8196a(12) =11507a(13) =15731a(14) =21022a(15) =27546a(16) =35481a(17) =45017a(18) =56356a(19) =69712a(20) =85311a(21) =103391a(22) =124202a(23) =148006a(24) =175077a(25) =205701a(26) =240176a(27) =278812a(28) =321931a(29) =369867