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 A050717 Inserting a digit '7' between adjacent digits of n makes a prime. 9
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS EXAMPLE E.g. 477 becomes 4(7)7(7)7 which is prime 47777. MATHEMATICA Select[Range[10, 500], PrimeQ[FromDigits[Riffle[IntegerDigits[#], 7]]]&] (* Georg Fischer, Oct 15 2019 after Harvey P. Dale *) CROSSREFS Cf. A050674, A050711, A050712, A050713, A050714, A050716, A050717, A050718, A050719. Sequence in context: A216639 A171098 A118845 * A335034 A188540 A088184 Adjacent sequences:  A050714 A050715 A050716 * A050718 A050719 A050720 KEYWORD nonn,base AUTHOR Patrick De Geest, Aug 15 1999 EXTENSIONS Offset changed to 1 by Georg Fischer, Oct 15 2019 STATUS approved

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Last modified June 16 14:16 EDT 2021. Contains 345057 sequences. (Running on oeis4.)