OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 13 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section).
a(21) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 130w1.
EXAMPLE
2 is in this sequence because 13*10^3 + 1 = 1301 is prime.
Initial terms and associated primes:
a(1) = 1, 131;
a(2) = 2, 1301;
a(3) = 3, 13001;
a(4) = 7, 130000001;
a(5) = 16, 130000000000000001; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[13*10^# + 1] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 22 2017
STATUS
approved