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 A050711 Inserting a digit '1' between adjacent digits of n makes a prime. 20
 13, 21, 31, 33, 37, 49, 63, 67, 69, 79, 81, 91, 99, 113, 117, 119, 123, 131, 137, 141, 159, 167, 177, 179, 183, 201, 203, 207, 209, 221, 233, 237, 239, 249, 257, 261, 263, 267, 273, 287, 291, 303, 309, 329, 339, 351, 353, 357, 387, 401, 407, 413, 417, 423 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1001 EXAMPLE 303 becomes 3(1)0(1)3 which is prime 31013. MATHEMATICA Select[Range[10, 500], PrimeQ[FromDigits[Riffle[IntegerDigits[#], 1]]]&] (* Harvey P. Dale, Apr 25 2019 *) PROG (PARI) is(n) = if(gcd(10, n%10) > 1 || n < 10, return(0)); my(d = digits(n), v = vector(2*#d - 1, i, if(i % 2 == 1, d[i>>1 + 1], 1))); isprime(fromdigits(v)) \\ David A. Corneth, Oct 15 2019 CROSSREFS Cf. A050674, A050712, A050713, A050714, A050715, A050716, A050717, A050718, A050719. Sequence in context: A128819 A165955 A331116 * A282844 A020440 A164458 Adjacent sequences:  A050708 A050709 A050710 * A050712 A050713 A050714 KEYWORD nonn,base AUTHOR Patrick De Geest, Aug 15 1999 EXTENSIONS Offset set to 1 by Georg Fischer, Oct 15 2019 STATUS approved

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Last modified January 29 00:42 EST 2022. Contains 350670 sequences. (Running on oeis4.)