OFFSET
1,1
COMMENTS
The ratio (Sum_(n=1..t) a(n)) / (Sum_(n=1..t) n) tends to log(5) as t increases.
LINKS
Pierre CAMI, Table of n, a(n) for n = 1..2999
MATHEMATICA
Array[Block[{k = 2}, While[Or[Divisible[k, 5], ! PrimeQ[k 5^# + 1]], k++]; k] &, 60] (* Michael De Vlieger, Dec 18 2017 *)
PROG
(PARI) a(n) = {k = 1; while (!isprime(k*5^n+1), k++; if (!(k%5), k++)); k; } \\ Michel Marcus, Nov 25 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Pierre CAMI, Nov 25 2017
STATUS
approved