Concentric 21-gonal numbers.

Terms

a(0) =0a(1) =1a(2) =21a(3) =43a(4) =84a(5) =127a(6) =189a(7) =253a(8) =336a(9) =421a(10) =525a(11) =631a(12) =756a(13) =883a(14) =1029a(15) =1177a(16) =1344a(17) =1513a(18) =1701a(19) =1891a(20) =2100a(21) =2311a(22) =2541a(23) =2773a(24) =3024a(25) =3277a(26) =3549a(27) =3823a(28) =4116a(29) =4411