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A056693 Numbers n such that 70*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
1, 65, 85, 89, 101, 385, 623, 7783, 18535 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that (7*10^(n+1)+11)/9 is prime.

LINKS

Table of n, a(n) for n=1..9.

Makoto Kamada, Prime numbers of the form 77...779.

Index entries for primes involving repunits

FORMULA

a(n) = A098089(n+1) - 1. - Robert Price, Nov 22 2014

MATHEMATICA

Do[ If[ PrimeQ[70*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}]

Select[Range[700], PrimeQ[(7 10^(# + 1) + 11) / 9] &] (* Vincenzo Librandi, Nov 22 2014 *)

PROG

(MAGMA) [n: n in [1..400] |  IsPrime((7*10^(n+1)+11) div 9)];  Vincenzo Librandi, Nov 22 2014

CROSSREFS

Cf. A002275, A093404, A098089.

Sequence in context: A113688 A214484 A159758 * A164282 A025312 A024508

Adjacent sequences:  A056690 A056691 A056692 * A056694 A056695 A056696

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 10 2000

EXTENSIONS

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

a(9) derived from A098089 by Robert Price, Nov 22 2014

STATUS

approved

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Last modified October 18 04:14 EDT 2018. Contains 316304 sequences. (Running on oeis4.)