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Primes that yield another prime on placing a 7 on both sides (as leading and trailing digits).
7

%I #6 Dec 05 2013 19:55:17

%S 2,5,17,23,29,41,47,53,71,131,191,227,233,257,281,293,347,359,389,401,

%T 419,431,479,503,521,557,593,599,653,659,677,683,701,761,797,827,857,

%U 887,953,977,1091,1097,1109,1151,1181,1187,1193,1223,1283,1307,1373

%N Primes that yield another prime on placing a 7 on both sides (as leading and trailing digits).

%H Harvey P. Dale, <a href="/A069689/b069689.txt">Table of n, a(n) for n = 1..1000</a>

%e 233 is a member as 72337 is also a prime.

%t Select[ Range[2000], PrimeQ[ # ] && PrimeQ[ FromDigits[ Insert[ IntegerDigits[ # ], 7, {{1}, {-1}}]]] &]

%t Select[Prime[Range[250]],PrimeQ[FromDigits[Join[{7}, IntegerDigits[ #],{7}]]]&] (* _Harvey P. Dale_, Nov 01 2011 *)

%Y Cf. A069687, A069688 & A069690.

%K nonn,base

%O 1,1

%A _Amarnath Murthy_, Apr 06 2002

%E Edited and extended by _Robert G. Wilson v_, May 03 2002