OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 71 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 23w71.
EXAMPLE
2 is in this sequence because (7*10^2 + 113)/3 = 271 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 2, 271;
a(3) = 3, 2371;
a(4) = 4, 23371;
a(5) = 5, 233371; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(7*10^# + 113)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 12 2017
EXTENSIONS
a(22)-a(24) from Robert Price, Feb 21 2018
STATUS
approved