OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 6 followed by the digits 51 is prime (see Example section).
a(26) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 76w51.
EXAMPLE
4 is in this sequence because (23*10^4 - 47)/3 = 76651 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 2, 751;
a(3) = 4, 76651;
a(4) = 5, 766651;
a(5) = 11, 766666666651; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(23*10^# - 47)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 04 2017
EXTENSIONS
a(24) from Robert Price, Sep 22 2019
a(25) from Robert Price, Oct 25 2023
STATUS
approved