a(n) = pg(3, n) + pg(4, n) + ... + pg(n, n) where pg(m, n) is the n-th m-th-order polygonal number.

Terms

a(0) =0a(1) =0a(2) =0a(3) =6a(4) =26a(5) =75a(6) =174a(7) =350a(8) =636a(9) =1071a(10) =1700a(11) =2574a(12) =3750a(13) =5291a(14) =7266a(15) =9750a(16) =12824a(17) =16575a(18) =21096a(19) =26486a(20) =32850a(21) =40299a(22) =48950a(23) =58926a(24) =70356a(25) =83375a(26) =98124a(27) =114750a(28) =133406a(29) =154251