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A096506 Numbers n for which 2*R_n + 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n. 6
1, 2, 3, 8, 11, 36, 95, 101, 128, 260, 351, 467, 645, 1011, 1178, 1217, 2442, 3761, 3806, 15617, 26459, 63117, 88545, 93497 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that (2*10^n + 7)/9 is prime.

LINKS

Table of n, a(n) for n=1..24.

Makoto Kamada, Prime numbers of the form 22...223.

Index entries for primes involving repunits

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

Cf. A056656, A093162, A096503, A096504, A096505, A096507, A096508.

Sequence in context: A041123 A042045 A317912 * A119155 A119195 A119199

Adjacent sequences:  A096503 A096504 A096505 * A096507 A096508 A096509

KEYWORD

nonn

AUTHOR

Labos Elemer, Jul 12 2004

EXTENSIONS

a(20)-a(24) from Kamada link by Ray Chandler, Feb 27 2012

STATUS

approved

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Last modified August 6 12:33 EDT 2020. Contains 336246 sequences. (Running on oeis4.)