OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 0 followed by the digits 51 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 20w51.
EXAMPLE
2 is in this sequence because 2*10^2 + 51 = 251 is prime.
Initial terms and associated primes:
a(1) = 0, 53;
a(2) = 1, 71;
a(3) = 2, 251;
a(4) = 4, 20051;
a(5) = 8, 200000051; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[2*10^# + 51] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 19 2017
EXTENSIONS
a(25)-a(27) from Robert Price, May 30 2018
STATUS
approved