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

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

1,1

Comments

Alternatively: Semiprimes p*q, with p<q, such that the concatenation p || q is a prime.

Corresponding primes are at A105184.

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Example

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.

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 *)

Prog

(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

Crossrefs

Cf. A000040, A001358, A100607, A105184.

Sequence in context: A200831 A184291 A102993 * A143416 A273787 A323916

Adjacent sequences: A276800 A276801 A276802 * A276804 A276805 A276806

Keyword

nonn,base

Author

K. D. Bajpai, Sep 17 2016

Status

approved