Number of 9-element subsets of [n] whose sum is a triangular number.

Terms

a(0) =1a(1) =1a(2) =6a(3) =21a(4) =64a(5) =174a(6) =419a(7) =930a(8) =1921a(9) =3741a(10) =6933a(11) =12303a(12) =21044a(13) =34839a(14) =56044a(15) =87885a(16) =134682a(17) =202181a(18) =297873a(19) =431423a(20) =615166a(21) =864673a(22) =1199421a(23) =1643537a(24) =2226716a(25) =2985188a(26) =3962845a(27) =5212562a(28) =6797615a(29) =8793297