a(n) = bc, where n = C(b,2)+C(c,1), b>c>=0.

Terms

a(0) =20a(1) =21a(2) =30a(3) =31a(4) =32a(5) =40a(6) =41a(7) =42a(8) =43a(9) =50a(10) =51a(11) =52a(12) =53a(13) =54a(14) =60a(15) =61a(16) =62a(17) =63a(18) =64a(19) =65a(20) =70a(21) =71a(22) =72a(23) =73a(24) =74a(25) =75a(26) =76a(27) =80a(28) =81a(29) =82