OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 4 followed by the digits 57 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 84w57.
EXAMPLE
2 is in this sequence because (76*10^2 + 113)/9 = 857 is prime.
Initial terms and associated primes:
a(1) = 1, 97;
a(2) = 2, 857;
a(3) = 4, 84457;
a(4) = 5, 844457;
a(5) = 8, 844444457; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(76*10^# + 113)/9] &] (* Corrected by Georg Fischer, Jul 22 2019 *)
PROG
(PARI) isok(k) = isprime((76*10^k + 113)/9); \\ Michel Marcus, Nov 12 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 11 2017
STATUS
approved