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A056695 Numbers n such that 80*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
0, 2, 3, 5, 8, 11, 71, 117, 123, 189, 243, 303, 356, 1421, 2690, 5537, 7580, 21905, 32175, 44357 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that (8*10^(n+1)-17)/9 is prime.

a(21) > 10^5. - Robert Price, May 20 2014

LINKS

Table of n, a(n) for n=1..20.

Makoto Kamada, Prime numbers of the form 88...887.

Index entries for primes involving repunits

FORMULA

a(n) = A096846(n) - 1.

MATHEMATICA

Do[ If[ PrimeQ[80*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]

CROSSREFS

Cf. A002275, A093171, A096846.

Sequence in context: A259973 A092362 A105766 * A287006 A020899 A057987

Adjacent sequences:  A056692 A056693 A056694 * A056696 A056697 A056698

KEYWORD

hard,nonn,more

AUTHOR

Robert G. Wilson v, Aug 10 2000

EXTENSIONS

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(18)-a(20) derived from A096846 by Robert Price, May 20 2014

STATUS

approved

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Last modified October 1 19:29 EDT 2020. Contains 337444 sequences. (Running on oeis4.)