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%I #16 Aug 19 2014 00:51:43
%S 9,21,41,101,143,171,249,251,297,329,333,347,357,489,509,531,537,557,
%T 569,611,629,653,657,669,731,773,821,843,861,941,957,983,993,1133,
%U 1143,1209,1229,1239,1313,1361,1409,1427,1451,1497,1539,1647,1673,1791,1833
%N Numbers n such that concatenating 1 with five instances of n produces a prime.
%H Jens Kruse Andersen, <a href="/A242990/b242990.txt">Table of n, a(n) for n = 1..10000</a>
%e 143 is included because 1143143143143143 is a prime.
%t cQ[n_,i_]:=Module[{idn=IntegerDigits[n]},PrimeQ[FromDigits[Flatten[Join[ {1},Table[ idn,{i}]]]]]]; Select[Range[2000],cQ[#,5]&]
%o (Python)
%o from sympy import isprime
%o for n in range(2*10**3):
%o ..if isprime(int('1'+5*str(n))):
%o ....print(n,end=', ')
%o # _Derek Orr_, Aug 17 2014
%o (PARI) s=[]; for(n=1, 10^4, d=length(Str(n)); if(isprime(10^(5*d)+(10^(5*d)-1)/(10^d-1)*n), s=concat(s, n))); s \\ _Jens Kruse Andersen_, Aug 18 2014
%Y Cf. A242987, A242988, A242989.
%K nonn,base,easy
%O 1,1
%A _Harvey P. Dale_, Aug 17 2014
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