OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 6 followed by the digits 77 is prime (see Example section).
a(29) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 86w77.
EXAMPLE
3 is in this sequence because (26*10^3 + 31)/3 = 877 is prime.
Initial terms and associated primes:
a(1) = 0, 19;
a(2) = 1, 97;
a(3) = 2, 877;
a(4) = 3, 8677;
a(5) = 4, 86677, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(26*10^# + 31)/3] &]
PROG
(PARI) is(n)=ispseudoprime((26*10^n+31)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Jul 14 2016
STATUS
approved