OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 9 followed by the digits 21 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 79w21.
EXAMPLE
5 is in this sequence because 8*10^5 - 79 = 799921 is prime.
Initial terms and associated primes:
a(1) = 5, 799921;
a(2) = 6, 7999921;
a(3) = 13, 79999999999921;
a(4) = 28, 79999999999999999999999999921; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[8*10^# - 79] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 02 2017
STATUS
approved