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A217124
Semiprimes whose decimal representation has only digits in {4,5,7}.
1
4, 55, 57, 74, 77, 445, 447, 454, 545, 554, 745, 755, 4474, 4555, 4574, 4577, 4747, 4754, 4757, 4777, 5447, 5455, 5545, 5554, 5747, 5755, 5774, 5777, 7445, 7447, 7454, 7555, 7745, 7747, 7754, 44477, 44554, 44557, 44747, 44755, 45447, 45454, 45455, 45457
OFFSET
1,1
COMMENTS
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).
LINKS
FORMULA
A001358 INTERSECTION A214584.
EXAMPLE
4555 = 5 * 911 is semiprime.
MATHEMATICA
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 *)
Select[Flatten[Table[FromDigits/@Tuples[{4, 5, 7}, n], {n, 5}]], PrimeOmega[ #] == 2&] (* Harvey P. Dale, Sep 21 2016 *)
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Jonathan Vos Post, Sep 26 2012
EXTENSIONS
Corrected and extended by T. D. Noe, Sep 27 2012
STATUS
approved