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A099411
Numbers k such that 3*R_k + 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
4
1, 2, 3, 6, 46, 394, 978, 2586, 2811, 2968, 3642, 4827, 4918, 5592, 5706, 10683, 12891, 14118, 74350, 88680, 162138, 279978
OFFSET
1,2
COMMENTS
Also numbers k such that (10^k + 11)/3 is prime.
a(21) > 10^5. - Robert Price, Nov 02 2014
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