OFFSET
1,3
COMMENTS
For k > 0, numbers k such that the digits 18 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section).
a(26) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 180w1.
EXAMPLE
2 is in this sequence because 18*10^2 + 1 = 1801 is prime.
Initial terms and associated primes:
a(1) = 0, 19;
a(2) = 1, 181;
a(3) = 2, 1801;
a(4) = 4, 180001;
a(5) = 9, 18000000001; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[18*10^# + 1] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 15 2017
STATUS
approved