Multiples of 21.

Terms

a(0) =0a(1) =21a(2) =42a(3) =63a(4) =84a(5) =105a(6) =126a(7) =147a(8) =168a(9) =189a(10) =210a(11) =231a(12) =252a(13) =273a(14) =294a(15) =315a(16) =336a(17) =357a(18) =378a(19) =399a(20) =420a(21) =441a(22) =462a(23) =483a(24) =504a(25) =525a(26) =546a(27) =567a(28) =588a(29) =609