OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 1 followed by the digits 61 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 91w61.
EXAMPLE
5 is in this sequence because (82*10^5 + 449)/9 = 911161 is prime.
Initial terms and associated primes:
a(1) = 0, 59;
a(2) = 3, 9161;
a(3) = 5, 911161;
a(4) = 6, 9111161;
a(5) = 18, 9111111111111111161; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(82*10^# + 449)/9] &] (* Corrected by Georg Fischer, Jul 22 2019 *)
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 22 2017
EXTENSIONS
a(25) from Robert Price, Nov 09 2019
STATUS
approved