OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 2 followed by the digits 57 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 82w57.
EXAMPLE
2 is in this sequence because (74*10^2 + 313)/9 = 857 is prime.
Initial terms and associated primes:
a(1) = 0, 43;
a(2) = 2, 857;
a(3) = 6, 8222257;
a(4) = 15, 8222222222222257;
a(5) = 18, 8222222222222222257; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(74*10^# + 313)/9] &]
PROG
(PARI) isok(k) = isprime((74*10^k + 313)/9); \\ Michel Marcus, Nov 12 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 11 2017
EXTENSIONS
a(24) from Robert Price, Jun 14 2019
STATUS
approved