Numbers of the form 3^i*7^j with i, j >= 0.

Terms

a(0) =1a(1) =3a(2) =7a(3) =9a(4) =21a(5) =27a(6) =49a(7) =63a(8) =81a(9) =147a(10) =189a(11) =243a(12) =343a(13) =441a(14) =567a(15) =729a(16) =1029a(17) =1323a(18) =1701a(19) =2187a(20) =2401a(21) =3087a(22) =3969a(23) =5103a(24) =6561a(25) =7203a(26) =9261a(27) =11907a(28) =15309a(29) =16807