A069689 Primes that yield another prime on placing a 7 on both sides (as leading and trailing digits).

2, 5, 17, 23, 29, 41, 47, 53, 71, 131, 191, 227, 233, 257, 281, 293, 347, 359, 389, 401, 419, 431, 479, 503, 521, 557, 593, 599, 653, 659, 677, 683, 701, 761, 797, 827, 857, 887, 953, 977, 1091, 1097, 1109, 1151, 1181, 1187, 1193, 1223, 1283, 1307, 1373

Offset: 1

Author

Amarnath Murthy, Apr 06 2002

Keywords

• nonn
• base

Examples

			233 is a member as 72337 is also a prime.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A069687, A069688 & A069690.

Programs

Mathematica

Select[ Range[2000], PrimeQ[ # ] && PrimeQ[ FromDigits[ Insert[ IntegerDigits[ # ], 7, {{1}, {-1}}]]] &]
Select[Prime[Range[250]],PrimeQ[FromDigits[Join[{7}, IntegerDigits[ #],{7}]]]&]  (* Harvey P. Dale, Nov 01 2011 *)

Extensions

Edited and extended by Robert G. Wilson v, May 03 2002