Composite numbers whose prime factors contain no digits other than 7 and 8.

Terms

a(0) =49a(1) =343a(2) =2401a(3) =5509a(4) =6139a(5) =6209a(6) =16807a(7) =38563a(8) =42973a(9) =43463a(10) =55139a(11) =62209a(12) =117649a(13) =269941a(14) =300811a(15) =304241a(16) =385973a(17) =435463a(18) =551509a(19) =552139a(20) =552209a(21) =615139a(22) =615209a(23) =619369a(24) =690199a(25) =698069a(26) =769129a(27) =777899a(28) =786769a(29) =823543