OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 71 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 26w71.
EXAMPLE
2 is in this sequence because (8*10^2 + 13)/3 = 271 is prime.
Initial terms and associated primes:
a(1) = 0, 7;
a(2) = 1, 31;
a(3) = 2, 271;
a(4) = 3, 2671;
a(5) = 5, 266671; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(8*10^# + 13)/3] &]
PROG
(PARI) isok(k) = isprime((8*10^k + 13)/3); \\ Michel Marcus, Nov 12 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 11 2017
EXTENSIONS
a(22)-a(23) from Robert Price, Jun 12 2018
STATUS
approved