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 87 is prime (see Example section).
a(29) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 86w87.
EXAMPLE
2 is in this sequence because (26*10^2 + 61)/3 = 887 is prime.
Initial terms and associated primes:
a(1) = 0, 29;
a(2) = 1, 107;
a(3) = 2, 887;
a(4) = 8, 866666687;
a(5) = 10, 86666666687; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(26*10^# + 61)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 17 2017
EXTENSIONS
a(28) from Robert Price, Oct 13 2019
STATUS
approved