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A096845
Numbers n for which 4*R_n - 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.
3
1, 2, 3, 6, 9, 12, 30, 32, 183, 297, 492, 41316
OFFSET
1,2
COMMENTS
Also numbers n such that (4*10^n-13)/9 is prime.
a(13) > 10^5. - Robert Price, Oct 25 2014
FORMULA
a(n) = A056661(n) + 1.
EXAMPLE
n=30 means that 444444444444444444444444444443 is prime.
MATHEMATICA
Do[ If[ PrimeQ[ 4(10^n - 1)/9 - 1], Print[n]], {n, 5000}] (* Robert G. Wilson v, Oct 14 2004 *)
Select[Range[500], PrimeQ[FromDigits[PadLeft[{3}, #, 4]]]&] (* The program generates the first 11 terms of the sequence. *) (* Harvey P. Dale, Feb 10 2022 *)
KEYWORD
hard,more,nonn
AUTHOR
Labos Elemer, Jul 15 2004
EXTENSIONS
a(12) from Robert Price, Oct 25 2014
STATUS
approved