A217124 Semiprimes whose decimal representation has only digits in {4,5,7}.

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

Harvey P. Dale, Table of n, a(n) for n = 1..1000

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

Cf. A001358, A072960, A214584.

Sequence in context: A140604 A048371 A077658 * A064439 A352510 A133218

Adjacent sequences: A217121 A217122 A217123 * A217125 A217126 A217127

Keyword

nonn,base,easy

Author

Jonathan Vos Post, Sep 26 2012

Extensions

Corrected and extended by T. D. Noe, Sep 27 2012

Status

approved