OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digits 12 followed by k-1 occurrences of the digit 1 followed by the digit 3 is prime (see Example section).
a(20) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 121w3.
EXAMPLE
2 is in this sequence because (109*10^2 + 17)/9 = 1213 is prime.
Initial terms and associated primes:
a(1) = 2, 1213;
a(2) = 3, 12113;
a(3) = 9, 12111111113;
a(4) = 23, 1211111111111111111111113;
a(5) = 27, 12111111111111111111111111113; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(109*10^# + 17)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 10 2017
STATUS
approved