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A056719 Numbers n such that 7*10^n + R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1
0, 1, 7, 55, 83461, 762811 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Also numbers n such that (64*10^n-1)/9 is prime.

No other terms below 820000. - Serge Batalov, Sep 22 2014

LINKS

Table of n, a(n) for n=1..6.

Makoto Kamada, Prime numbers of the form 711...11.

Index entries for primes involving repunits

MATHEMATICA

Do[ If[ PrimeQ[ 7*10^n + (10^n-1)/9], Print[n]], {n, 0, 35076}]

CROSSREFS

Cf. A002275, A093632.

Sequence in context: A197228 A009521 A116120 * A038855 A097254 A043911

Adjacent sequences:  A056716 A056717 A056718 * A056720 A056721 A056722

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 11 2000

EXTENSIONS

A new PRP term from Serge Batalov, Aug 13 2009

A new PRP term 762811 from Serge Batalov, Sep 22 2014

STATUS

approved

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Last modified October 15 03:16 EDT 2019. Contains 328025 sequences. (Running on oeis4.)