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A056655 Numbers n such that 10*R_n +7 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
0, 1, 3, 4, 7, 22, 28, 39, 130, 135, 214, 610, 766, 2152, 2575, 22972, 42688, 85711, 85863 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Also numbers n such that (10^(n+1)+53)/9 is prime.

2575 also produces a probable prime.

a(20) > 10^5. - Robert Price, Jan 13 2015

LINKS

Table of n, a(n) for n=1..19.

Makoto Kamada, Prime numbers of the form 11...117.

Index entries for primes involving repunits

FORMULA

a(n) = A097684(n) - 1 for all n >= 0. - Rick L. Shepherd, Aug 23 2004

MATHEMATICA

Do[ m = n; If[ primeQ[ 10*(10^n - 1)/9 + 7 ], Print[ n ] ], {n, 1, 1250} ]

CROSSREFS

Cf. A093139 (corresponding primes), A097684.

Sequence in context: A241660 A030724 A124082 * A332971 A292033 A288501

Adjacent sequences:  A056652 A056653 A056654 * A056656 A056657 A056658

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 09 2000

EXTENSIONS

2152 (giving a probable prime) from Rick L. Shepherd, Mar 23 2004

2575 from Rick L. Shepherd, Aug 23 2004

a(16)-a(19) derived from A097684 by Robert Price, Jan 13 2015

STATUS

approved

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Last modified May 26 02:56 EDT 2020. Contains 334613 sequences. (Running on oeis4.)