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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A276803 Semiprimes k such that the concatenation of its prime factors is prime.

Original entry on oeis.org

6, 21, 22, 33, 39, 46, 51, 58, 82, 93, 111, 115, 133, 141, 142, 159, 166, 177, 187, 201, 205, 219, 226, 235, 237, 247, 249, 253, 262, 267, 274, 291, 301, 319, 327, 355, 358, 391, 411, 427, 478, 489, 501, 502, 505, 511, 535, 538, 543, 562, 565, 573, 583, 586, 589
Offset: 1

Views

Author

K. D. Bajpai, Sep 17 2016

Keywords

Comments

Alternatively: Semiprimes p*q, with p
Corresponding primes are at A105184.

Examples

			21 is a term because 21 = 3 * 7 that is a semiprime : concatenation of 3 and 7 = 37  which is prime.
142 is a term because 142 = 2 * 71 that is a semiprime : concatenation of 2 and 71 = 271 which is prime.

Links

Crossrefs

Cf. A000040, A001358, A100607, A105184.

Programs

  • Mathematica
    Select[Select[Range[1000], PrimeOmega[#] == 2 &], PrimeQ[FromDigits[Join[IntegerDigits [First@First[FactorInteger[#]]], IntegerDigits[First@Last[FactorInteger[#]]]]]] &]
Select[Range[1000],PrimeOmega[#]==PrimeNu[#]==2&&PrimeQ[FromDigits[ Flatten[ IntegerDigits/@FactorInteger[#][[All,1]]]]]&] (* Harvey P. Dale, Aug 03 2022 *)
  • PARI
    list(lim)=my(v=List()); forprime(p=2,lim\2, forprime(q=2, min(p,lim\p), if(isprime(eval(Str(q,p))), listput(v,p*q)))); Set(v) \\ Charles R Greathouse IV, Sep 17 2016

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