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%I #18 Dec 27 2017 03:17:47
%S 2,4,2,18,12,4,12,24,26,18,42,28,2,16,18,34,92,4,12,46,26,16,6,36,26,
%T 16,6,16,152,18,42,136,6,162,132,58,24,142,48,22,56,16,36,84,2,12,24,
%U 108,168,4,12,214,36,114,12,444,26,54,416,42
%N Smallest k not divisible by 5 such that k*5^n + 1 is prime.
%C The ratio (Sum_(n=1..t) a(n)) / (Sum_(n=1..t) n) tends to log(5) as t increases.
%H Pierre CAMI, <a href="/A295640/b295640.txt">Table of n, a(n) for n = 1..2999</a>
%t Array[Block[{k = 2}, While[Or[Divisible[k, 5], ! PrimeQ[k 5^# + 1]], k++]; k] &, 60] (* _Michael De Vlieger_, Dec 18 2017 *)
%o (PARI) a(n) = {k = 1; while (!isprime(k*5^n+1), k++; if (!(k%5), k++)); k;} \\ _Michel Marcus_, Nov 25 2017
%Y Cf. A057778, A295639, A295641.
%K nonn
%O 1,1
%A _Pierre CAMI_, Nov 25 2017