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%I #12 Sep 21 2016 11:58:20
%S 4,55,57,74,77,445,447,454,545,554,745,755,4474,4555,4574,4577,4747,
%T 4754,4757,4777,5447,5455,5545,5554,5747,5755,5774,5777,7445,7447,
%U 7454,7555,7745,7747,7754,44477,44554,44557,44747,44755,45447,45454,45455,45457
%N Semiprimes whose decimal representation has only digits in {4,5,7}.
%C 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 Harvey P. Dale, <a href="/A217124/b217124.txt">Table of n, a(n) for n = 1..1000</a>
%F A001358 INTERSECTION A214584.
%e 4555 = 5 * 911 is semiprime.
%t 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 Select[Flatten[Table[FromDigits/@Tuples[{4,5,7},n],{n,5}]],PrimeOmega[ #] == 2&] (* _Harvey P. Dale_, Sep 21 2016 *)
%Y Cf. A001358, A072960, A214584.
%K nonn,base,easy
%O 1,1
%A _Jonathan Vos Post_, Sep 26 2012
%E Corrected and extended by _T. D. Noe_, Sep 27 2012
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