a(n) = (-4*n^3 + 27*n^2 - 20*n + 3)/3.

Terms

a(0) =1a(1) =2a(2) =13a(3) =26a(4) =33a(5) =26a(6) =-3a(7) =-62a(8) =-159a(9) =-302a(10) =-499a(11) =-758a(12) =-1087a(13) =-1494a(14) =-1987a(15) =-2574a(16) =-3263a(17) =-4062a(18) =-4979a(19) =-6022a(20) =-7199a(21) =-8518a(22) =-9987a(23) =-11614a(24) =-13407a(25) =-15374a(26) =-17523a(27) =-19862a(28) =-22399a(29) =-25142