OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 80 followed by k-1 occurrences of the digit 8 followed by the digit 1 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 808w1.
EXAMPLE
2 is in this sequence because (728*10^2 - 71)/9 = 8081 is prime.
Initial terms and associated primes:
a(1) = 0, 73;
a(2) = 2, 8081;
a(3) = 5, 8088881;
a(4) = 6, 80888881;
a(5) = 11, 8088888888881; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(728*10^# - 71)/9] &]
PROG
(PARI) isok(k) = isprime((728*10^k - 71)/9); \\ Altug Alkan, Oct 08 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 08 2017
STATUS
approved