Composite numbers whose prime factors contain no digits other than 1 and 3.

Terms

a(0) =9a(1) =27a(2) =33a(3) =39a(4) =81a(5) =93a(6) =99a(7) =117a(8) =121a(9) =143a(10) =169a(11) =243a(12) =279a(13) =297a(14) =339a(15) =341a(16) =351a(17) =363a(18) =393a(19) =403a(20) =429a(21) =507a(22) =729a(23) =837a(24) =891a(25) =933a(26) =939a(27) =961a(28) =993a(29) =1017