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%I #9 Oct 15 2019 18:17:40
%S 13,19,21,27,33,39,49,51,57,63,67,73,87,91,97,107,137,141,147,153,159,
%T 191,197,203,207,219,221,227,249,263,273,279,311,323,327,339,351,353,
%U 359,381,389,429,477,479,497,503,507,513,519,521,533,551,569,573,593
%N Inserting a digit '7' between adjacent digits of n makes a prime.
%e E.g. 477 becomes 4(7)7(7)7 which is prime 47777.
%t Select[Range[10, 500], PrimeQ[FromDigits[Riffle[IntegerDigits[#], 7]]]&] (* _Georg Fischer_, Oct 15 2019 after Harvey P. Dale *)
%Y Cf. A050674, A050711, A050712, A050713, A050714, A050716, A050717, A050718, A050719.
%K nonn,base
%O 1,1
%A _Patrick De Geest_, Aug 15 1999
%E Offset changed to 1 by _Georg Fischer_, Oct 15 2019
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