A102500 Values of n for which the concatenation 7nn7 (from sequence A100897) are primes.

15, 18, 19, 34, 36, 39, 48, 52, 57, 60, 67, 75, 79, 81, 85, 96, 97, 1005, 1020, 1030, 1047, 1054, 1062, 1065, 1074, 1084, 1086, 1090, 1096, 1108, 1128, 1152, 1153, 1164, 1171, 1173, 1179, 1182, 1185, 1189, 1213, 1215, 1219, 1228, 1231, 1233, 1257, 1269, 1279

Offset: 1

Author

Mark Hudson (mrmarkhudson(AT)hotmail.com), Jan 13 2005

Keywords

• easy
• nonn
• base

Examples

			The number 715157 is prime and corresponds to n=15.
The number 7100510057 is prime and corresponds to n=1005.
		

Crossrefs

The full sequence of integers of the form 7nn7 is A100897. The primes that correspond to these values of n are sequence A102501.

Programs

Mathematica

fQ[n_] := Block[{id = IntegerDigits[n]}, PrimeQ[ FromDigits[ Join[{7}, id, id, {7}]]]]; Select[ Range[1290], fQ[ # ] &] (* Robert G. Wilson v, Jan 14 2004 *)

Extensions

More terms from Robert G. Wilson v, Jan 14 2005