Composite numbers whose prime factors contain no digits other than 2 and 7.

Terms

a(0) =4a(1) =8a(2) =14a(3) =16a(4) =28a(5) =32a(6) =49a(7) =56a(8) =64a(9) =98a(10) =112a(11) =128a(12) =196a(13) =224a(14) =256a(15) =343a(16) =392a(17) =448a(18) =454a(19) =512a(20) =554a(21) =686a(22) =784a(23) =896a(24) =908a(25) =1024a(26) =1108a(27) =1372a(28) =1454a(29) =1568