OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 89 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 26w89.
EXAMPLE
5 is in this sequence because (8*10^5 + 67)/3 = 2266689 7751 is prime.
Initial terms and associated primes:
a(1) = 3, 2689;
a(2) = 5, 266689;
a(3) = 9, 2666666689;
a(4) = 20, 266666666666666666689; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(8*10^# + 67)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 28 2017
STATUS
approved