OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 1 followed by the digits 29 is prime (see Example section).
a(27) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 21w29.
EXAMPLE
5 is in this sequence because (19*10^5 + 161)/9 = 211129 is prime.
Initial terms and associated primes:
a(1) = 2, 229;
a(2) = 3, 2129;
a(3) = 5, 211129;
a(4) = 6, 2111129;
a(5) = 8, 211111129, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(19*10^# + 161)/9] &]
PROG
(PARI) is(n)=ispseudoprime((19*10^n+161)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Jul 23 2016
STATUS
approved