Semiprimes having only straight digits.

Terms

a(0) =4a(1) =14a(2) =74a(3) =77a(4) =111a(5) =141a(6) =177a(7) =411a(8) =417a(9) =447a(10) =471a(11) =717a(12) =771a(13) =1111a(14) =1114a(15) =1141a(16) =1147a(17) =1174a(18) =1177a(19) =1411a(20) =1417a(21) =1441a(22) =1477a(23) =1711a(24) =1714a(25) =1717a(26) =1774a(27) =4117a(28) =4141a(29) =4171