OFFSET
1,2
COMMENTS
Also numbers n such that (2*10^n + 7)/9 is prime.
Per Kamada link, 181457, 202059, 262874 are also terms, found by Rytis Slatkevicius. - Michael S. Branicky, Sep 13 2024
LINKS
FORMULA
a(n) = A056656(n) + 1.
EXAMPLE
n=36: 222222222222222222222222222222222223 is a prime number.
MATHEMATICA
Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 1], Print[n]], {n, 7000}] (* Robert G. Wilson v, Oct 14 2004 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Jul 12 2004
EXTENSIONS
a(20)-a(24) from Kamada link by Ray Chandler, Feb 27 2012
STATUS
approved