OFFSET
1,1
COMMENTS
The corresponding primes are a subsequence of A065074: near-repunit primes that contain the digit 0.
In base 10, R(k) - 10^floor(k/2-1) has ceiling(k/2) digits 1, one digit 0 and again floor(k/2-1) digits 1: for even as well as odd k, there is a digit 0 just right of the middle of the repunit of length k.
No term can be congruent to 1 (mod 3). - Chai Wah Wu, Feb 07 2020
a(13) > 50000. - Michael S. Branicky, Jul 23 2024
EXAMPLE
For k = 8, R(8) - 10^(4-1) = 11110111 is prime.
For k = 12, R(12) - 10^(6-1) = 111111011111 is prime.
For k = 17, R(12) - 10^(8-1) = 11111111101111111 is prime.
PROG
(PARI) for(n=2, 9999, isprime(p=10^n\9-10^(n\2-1))&&print1(n", "))
CROSSREFS
KEYWORD
nonn,hard,more,base
AUTHOR
M. F. Hasler, Jan 30 2020
EXTENSIONS
a(7)-a(10) from Giovanni Resta, Jan 31 2020
a(11)-a(12) from Michael S. Branicky, Jul 22 2024
STATUS
approved