A217048 -id:A217048 - cp's OEIS frontend

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

Jonathan Vos Post, Sep 26 2012

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).

			4555 = 5 * 911 is semiprime.

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

Cf. A001358, A072960, A214584.

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 *)

A001358 INTERSECTION A214584.

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

cp's OEIS Frontend

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

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