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%I #18 Oct 15 2019 12:14:51
%S 13,21,31,33,37,49,63,67,69,79,81,91,99,113,117,119,123,131,137,141,
%T 159,167,177,179,183,201,203,207,209,221,233,237,239,249,257,261,263,
%U 267,273,287,291,303,309,329,339,351,353,357,387,401,407,413,417,423
%N Inserting a digit '1' between adjacent digits of n makes a prime.
%H Harvey P. Dale, <a href="/A050711/b050711.txt">Table of n, a(n) for n = 1..1001</a>
%e 303 becomes 3(1)0(1)3 which is prime 31013.
%t Select[Range[10,500],PrimeQ[FromDigits[Riffle[IntegerDigits[#],1]]]&] (* _Harvey P. Dale_, Apr 25 2019 *)
%o (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
%Y Cf. A050674, A050712, A050713, A050714, A050715, A050716, A050717, A050718, A050719.
%K nonn,base
%O 1,1
%A _Patrick De Geest_, Aug 15 1999
%E Offset set to 1 by _Georg Fischer_, Oct 15 2019
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