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%I #14 Oct 22 2016 14:46:44
%S 7,19,37,41,199,239,311,587,661,941,967,1009,1997,4993,4999,5393,5651,
%T 6911,9109,9397,9679,9829,19417,20233,22549,27397,29389,31387,39989,
%U 71419,71569,90599,91951,95369,97103,98909,99023,160009,225919,267389,313991,328849
%N Primes that remain prime when a single digit 9 is inserted between any two consecutive digits or as the leading or trailing digit.
%H Harvey P. Dale, <a href="/A215421/b215421.txt">Table of n, a(n) for n = 1..100</a>
%e 31387 is prime and also 313879, 313897, 313987, 319387, 391387, 931387.
%p A215421:=proc(q,x)
%p local a,b,c,d,i,n,ok;
%p for n from 1 to q do
%p a:=ithprime(n); b:=0; while a>0 do b:=b+1; a:=trunc(a/10); od;
%p a:=ithprime(n); ok:=1;
%p for i from 0 to b do
%p c:=a+9*10^i*trunc(a/10^i)+10^i*x; if not isprime(c) then ok:=0; break; fi;
%p od;
%p if ok=1 then print(ithprime(n)); fi;
%p od; end:
%p A215421(1000,9);
%t 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 *)
%Y Cf. A215417-A215420
%K nonn,base
%O 1,1
%A _Paolo P. Lava_, Aug 10 2012
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