OFFSET
1,2
COMMENTS
Also numbers k such that (10^k + 11)/3 is prime.
a(21) > 10^5. - Robert Price, Nov 02 2014
LINKS
FORMULA
a(n) = A056680(n) + 1.
MATHEMATICA
Do[ If[ PrimeQ[ 3(10^n - 1)/9 + 4], Print[n]], {n, 10000}]
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Oct 14 2004
EXTENSIONS
a(16)-a(20) from Robert Price, Nov 02 2014
a(21)-a(22) from Kamada data by Tyler Busby, May 03 2024
STATUS
approved