OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 2 followed by the digits 77 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 52w77.
EXAMPLE
2 is in this sequence because (47*10^2 + 493)/9 = 577 is prime.
Initial terms and associated primes:
a(1) = 1, 107;
a(2) = 2, 577;
a(3) = 7, 52222277;
a(4) = 10, 52222222277;
a(5) = 16, 52222222222222277; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(47*10^# + 493)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 28 2017
EXTENSIONS
a(23)-a(24) from Robert Price, Mar 23 2019
STATUS
approved