Triangular numbers of the form 21*k.

Terms

a(0) =0a(1) =21a(2) =105a(3) =210a(4) =231a(5) =378a(6) =630a(7) =861a(8) =903a(9) =1176a(10) =1596a(11) =1953a(12) =2016a(13) =2415a(14) =3003a(15) =3486a(16) =3570a(17) =4095a(18) =4851a(19) =5460a(20) =5565a(21) =6216a(22) =7140a(23) =7875a(24) =8001a(25) =8778a(26) =9870a(27) =10731a(28) =10878a(29) =11781