Tritriangular numbers: a(n) = binomial(binomial(n,2),2) = n*(n+1)*(n-1)*(n-2)/8.

Terms

a(0) =0a(1) =0a(2) =0a(3) =3a(4) =15a(5) =45a(6) =105a(7) =210a(8) =378a(9) =630a(10) =990a(11) =1485a(12) =2145a(13) =3003a(14) =4095a(15) =5460a(16) =7140a(17) =9180a(18) =11628a(19) =14535a(20) =17955a(21) =21945a(22) =26565a(23) =31878a(24) =37950a(25) =44850a(26) =52650a(27) =61425a(28) =71253a(29) =82215