OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 8 followed by the digits 33 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 28w33.
EXAMPLE
2 is in this sequence because (26*10^2 - 503)/9 = 233 is prime.
Initial terms and associated primes:
a(1) = 2, 233;
a(2) = 3, 2833;
a(3) = 5, 288833;
a(4) = 15, 2888888888888833;
a(5) = 23, 288888888888888888888833; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(26*10^# - 503)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 03 2017
EXTENSIONS
a(23)-a(25) from Robert Price, Jul 04 2018
STATUS
approved