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%I #7 Dec 05 2013 19:56:32
%S 1,2,1,1,3,100005,1,2,4,6,8,100010,19,2,215,9,60,100041,4,66,5,1,41,
%T 100061,4,15,2,1,195,100055,61,1061,143,12,72,100127,19,60,1,6,125,0,
%U 45,1305,3,39,27,100269,72,95,136,1123,50,100193,52,1056,176,1536,66
%N Smallest k such that n times concatenation of k with itself followed by a 7 is a prime, or 0 if no such number exists.
%C a(42n)=0, but all other terms are probably nonzero. For n a multiple of 42, (10^(l*n)-1)/(10^l-1)*10+7 is divisible by 7 for any l. - _Max Alekseyev_, Feb 11 2005
%F Minimal k such that k*(10^(l*n)-1)/(10^l-1)*10+7 is prime, where l is the length of k; and 0 if no such prime exists. - _Max Alekseyev_, Feb 11 2005
%e a(5) = 3 as 333337 is a prime but 111117 and 222227 are not.
%o (PARI) { a(n) = if(n%42==0,return(0)); for(l=1,10^6, if(valuation(10^(l*n)-1,7)==valuation(10^l-1,7), for(k=10^(l-1),10^l-1, if(isprime(k*(10^(l*n)-1)/(10^l-1)*10+7), return(k) ) ) ) ) } (Alekseyev)
%Y Cf. A086920, A087604, A087606, A087607, A087608, A087609, A087610.
%K base,nonn
%O 1,2
%A _Amarnath Murthy_, Sep 18 2003
%E Corrected and extended by _Max Alekseyev_, Feb 11 2005