This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 (list; graph; refs; listen; history; text; internal format)
 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 A133218 A190441 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified October 20 22:44 EDT 2019. Contains 328291 sequences. (Running on oeis4.)