Composite numbers whose prime factors have no digits other than 7's and 9's.

Terms

a(0) =49a(1) =343a(2) =553a(3) =679a(4) =2401a(5) =3871a(6) =4753a(7) =5579a(8) =6241a(9) =6839a(10) =6979a(11) =7663a(12) =9409a(13) =16807a(14) =27097a(15) =33271a(16) =39053a(17) =43687a(18) =47873a(19) =48853a(20) =53641a(21) =62963a(22) =65863a(23) =77183a(24) =77309a(25) =78763a(26) =94769a(27) =96709a(28) =117649a(29) =189679