a(n)=1+(1/12)(n*(n+1)*(n+3)*(4-n)).

Terms

a(0) =1a(1) =3a(2) =6a(3) =7a(4) =1a(5) =-19a(6) =-62a(7) =-139a(8) =-263a(9) =-449a(10) =-714a(11) =-1077a(12) =-1559a(13) =-2183a(14) =-2974a(15) =-3959a(16) =-5167a(17) =-6629a(18) =-8378a(19) =-10449a(20) =-12879a(21) =-15707a(22) =-18974a(23) =-22723a(24) =-26999a(25) =-31849a(26) =-37322a(27) =-43469a(28) =-50343a(29) =-57999