A215421 - OEIS

%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