A050717 Inserting a digit '7' between adjacent digits of n makes a prime.

13, 19, 21, 27, 33, 39, 49, 51, 57, 63, 67, 73, 87, 91, 97, 107, 137, 141, 147, 153, 159, 191, 197, 203, 207, 219, 221, 227, 249, 263, 273, 279, 311, 323, 327, 339, 351, 353, 359, 381, 389, 429, 477, 479, 497, 503, 507, 513, 519, 521, 533, 551, 569, 573, 593

Offset: 1

Author

Patrick De Geest, Aug 15 1999

Keywords

• nonn
• base

Examples

			E.g. 477 becomes 4(7)7(7)7 which is prime 47777.
		

Crossrefs

Cf. A050674, A050711, A050712, A050713, A050714, A050716, A050717, A050718, A050719.

Programs

Mathematica

Select[Range[10, 500], PrimeQ[FromDigits[Riffle[IntegerDigits[#], 7]]]&] (* Georg Fischer, Oct 15 2019 after Harvey P. Dale *)

Extensions

Offset changed to 1 by Georg Fischer, Oct 15 2019