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A215421
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Primes that remain prime when a single digit 9 is inserted between any two consecutive digits or as the leading or trailing digit.
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27
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7, 19, 37, 41, 199, 239, 311, 587, 661, 941, 967, 1009, 1997, 4993, 4999, 5393, 5651, 6911, 9109, 9397, 9679, 9829, 19417, 20233, 22549, 27397, 29389, 31387, 39989, 71419, 71569, 90599, 91951, 95369, 97103, 98909, 99023, 160009, 225919, 267389, 313991, 328849
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OFFSET
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1,1
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LINKS
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EXAMPLE
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31387 is prime and also 313879, 313897, 313987, 319387, 391387, 931387.
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MAPLE
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local a, b, c, d, i, n, ok;
for n from 1 to q do
a:=ithprime(n); b:=0; while a>0 do b:=b+1; a:=trunc(a/10); od;
a:=ithprime(n); ok:=1;
for i from 0 to b do
c:=a+9*10^i*trunc(a/10^i)+10^i*x; if not isprime(c) then ok:=0; break; fi;
od;
if ok=1 then print(ithprime(n)); fi;
od; end:
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MATHEMATICA
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Select[Prime[Range[30000]], AllTrue[FromDigits/@Table[Insert[ IntegerDigits[ #], 9, n], {n, IntegerLength[ #]+1}], PrimeQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Oct 22 2016 *)
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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