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A257460 Let b_k=7...7 consist of k>0 7's. Then a(n) is the smallest k such that the concatenation prime(n)b_k is prime, or a(n)=0 if there is no such prime.

%I A257460 #14 Jan 20 2021 18:38:44
%S A257460 2,1,2,0,3,1,2,1,2,48,1,10,2,3,3,3,9,1,1,2,66,1,2,8,1,2,6,3,1,3,1,2,3,
%T A257460 6,8,9,7,1,3,2,2,3,17,4,2,1,3,1,2,1,3,2,1,5,17,5,8,16,1,3,1,8,6,2,1,3,
%U A257460 3,2184,6,6,3,2,1,3,1,2,2,4,2,3,3,1,2,1,1,3,6,15,5,1,48,2,1,2,7,2,47,2,1,1
%N A257460 Let b_k=7...7 consist of k>0 7's. Then a(n) is the smallest k such that the concatenation prime(n)b_k is prime, or a(n)=0 if there is no such prime.
%C A257460 The only unknown terms less than 10000, tested to 17500, are for n: 484, 1291, 2096, 2238, 3503, 3859, 6674, 7087, 7824, 8954.
%H A257460 Vladimir Shevelev and Robert G. Wilson v, <a href="/A257460/a257460.txt">a(n) for n = 1..10000 with -1 for those entries where a(n) has not yet been found.</a>
%F A257460 a(n)=k for the least k such that prime(n)*10^k+7*(10^k-1)/9 is prime, where prime(n) is the n-th prime.
%t A257460 f[n_] := Block[{k = 1, p = Prime[n]}, While[ !PrimeQ[p*10^k + 7(10^k - 1)/9], k++]; k]; f[4] = 0; Array[f, 100]
%o A257460 (PARI) isok(k, dp) = ispseudoprime(fromdigits(concat(dp, vector(k, i, 7))));
%o A257460 a(n) = {if (prime(n) == 7, return(0)); my(k=1, p=prime(n)); while (!ispseudoprime(p*10^k+7*(10^k-1)/9), k++); k;} \\ _Michel Marcus_, Jan 20 2021
%Y A257460 Cf. A232210, A257459, A257461.
%K A257460 nonn,base
%O A257460 1,1
%A A257460 _Vladimir Shevelev_ and _Robert G. Wilson v_, Apr 24 2015