This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A217124 #12 Sep 21 2016 11:58:20
%S A217124 4,55,57,74,77,445,447,454,545,554,745,755,4474,4555,4574,4577,4747,
%T A217124 4754,4757,4777,5447,5455,5545,5554,5747,5755,5774,5777,7445,7447,
%U A217124 7454,7555,7745,7747,7754,44477,44554,44557,44747,44755,45447,45454,45455,45457
%N A217124 Semiprimes whose decimal representation has only digits in {4,5,7}.
%C A217124 Crooked semiprimes. This is to A217048 as integers all of whose numerals are written (san serif) with at least one right or acute angle (A214584) are to numbers using only the curved digits 0, 3, 6, 8 and 9 (A072960). This is to crooked primes (A217039) as semiprimes (A001358) are to primes (A000040).
%H A217124 Harvey P. Dale, <a href="/A217124/b217124.txt">Table of n, a(n) for n = 1..1000</a>
%F A217124 A001358 INTERSECTION A214584.
%e A217124 4555 = 5 * 911 is semiprime.
%t A217124 SemiPrimeQ[n_Integer] := If[Abs[n] < 2, False, (2 == Plus @@ Transpose[FactorInteger[Abs[n]]][[2]])]; Select[Flatten[Table[FromDigits /@ Tuples[{4, 5, 7}, n], {n, 5}]], SemiPrimeQ] (* _T. D. Noe_, Sep 27 2012 *)
%t A217124 Select[Flatten[Table[FromDigits/@Tuples[{4,5,7},n],{n,5}]],PrimeOmega[ #] == 2&] (* _Harvey P. Dale_, Sep 21 2016 *)
%Y A217124 Cf. A001358, A072960, A214584.
%K A217124 nonn,base,easy
%O A217124 1,1
%A A217124 _Jonathan Vos Post_, Sep 26 2012
%E A217124 Corrected and extended by _T. D. Noe_, Sep 27 2012