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%I #18 Dec 27 2017 03:16:22
%S 4,4,4,6,10,4,4,36,6,4,4,82,36,10,24,90,4,48,12,16,118,10,72,16,94,18,
%T 24,150,76,58,34,40,156,34,52,166,16,4,36,90,120,78,34,36,10,244,60,
%U 102,18,114,172,48,94,10,66,396,234,240,166
%N Smallest k not divisible by 7 such that k*7^n + 1 is prime.
%C The ratio (Sum_(n=1..t) a(n)) / (Sum_(n=1..t) n) tends to log(7) as t increases.
%H Pierre CAMI, <a href="/A295641/b295641.txt">Table of n, a(n) for n = 1..2000</a>
%t Array[Block[{k = 2}, While[Or[Divisible[k, 7], ! PrimeQ[k 7^# + 1]], k++]; k] &, 59] (* _Michael De Vlieger_, Dec 18 2017 *)
%o (PARI) a(n) = {k = 1; while (!isprime(k*7^n+1), k++; if (! (k%7), k++)); k;} \\ _Michel Marcus_, Nov 25 2017
%Y Cf. A057778, A295639, A295640.
%K nonn
%O 1,1
%A _Pierre CAMI_, Nov 25 2017