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A056721 Numbers n such that 8*10^n-1 is prime. 4
0, 1, 4, 5, 8, 10, 25, 49, 76, 128, 175, 238, 550, 796, 1219, 2012, 2846, 11336, 21296, 49808, 74318, 236891, 608989 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Also numbers n such that 7*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.

LINKS

Table of n, a(n) for n=1..23.

Makoto Kamada, Prime numbers of the form 799...99.

Index entries for primes involving repunits

MATHEMATICA

Do[ If[ PrimeQ[ 7*10^n + (10^n-1)], Print[n]], {n, 0, 3000}]

PROG

(PARI) is(n)=ispseudoprime(8*10^n-1) \\ Charles R Greathouse IV, Feb 20 2017

CROSSREFS

Sequence in context: A190394 A116050 A241413 * A057479 A050140 A190860

Adjacent sequences:  A056718 A056719 A056720 * A056722 A056723 A056724

KEYWORD

hard,nonn,more

AUTHOR

Robert G. Wilson v, Aug 11 2000

EXTENSIONS

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

a(22)-a(23) from Kamada data by Robert Price, Oct 19 2014

STATUS

approved

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Last modified April 20 06:19 EDT 2019. Contains 322294 sequences. (Running on oeis4.)