OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 11 is prime (see Example section).
a(15) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 50w11.
EXAMPLE
6 is in this sequence because 5*10^4 + 11 = 5000011 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 3, 5011;
a(3) = 6, 5000011;
a(4) = 35, 500000000000000000000000000000000011; etc.
MATHEMATICA
Select[Range[0, 200000], PrimeQ[5*10^# + 11] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 16 2019
STATUS
approved