OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 8 followed by the digits 79 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 28w79.
EXAMPLE
4 is in this sequence because (26*10^4 - 89) / 9 = 28879 is prime.
Initial terms and associated primes:
a(1) = 1, 19;
a(2) = 3, 2879;
a(3) = 4, 28879;
a(4) = 13, 28888888888879;
a(5) = 15, 2888888888888879; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(26*10^# - 89) / 9] &]
PROG
(PARI) is(n)=ispseudoprime((26*10^n-89)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 21 2016
EXTENSIONS
a(25)-a(26) from Robert Price, Jun 16 2018
STATUS
approved