OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 8 followed by the digits 57 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 78w57.
EXAMPLE
4 is in this sequence because (71*10^4 - 287)/9 = 78857 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 2, 757;
a(3) = 4, 78857;
a(4) = 8, 788888857;
a(5) = 26, 788888888888888888888888857; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(71*10^# - 287)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 16 2017
EXTENSIONS
a(25)-a(26) from Robert Price, Sep 06 2019
STATUS
approved