COMPOSE triangular numbers with triangular numbers.

Terms

a(0) =1a(1) =6a(2) =30a(3) =137a(4) =588a(5) =2415a(6) =9600a(7) =37209a(8) =141336a(9) =528049a(10) =1945800a(11) =7086555a(12) =25550547a(13) =91319973a(14) =323890062a(15) =1140985116a(16) =3995155506a(17) =13913280006a(18) =48216776266a(19) =166355744244a(20) =571636515369