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A056683 Numbers n such that 40*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275). 1
2, 4, 5, 47, 107, 244, 1043, 20207, 52739, 89188 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also numbers n such that (4*10^(n+1)+41)/9 is prime.

a(11) > 10^5. - Robert Price, Nov 09 2014

LINKS

Table of n, a(n) for n=1..10.

Index entries for primes involving repunits

FORMULA

a(n) = A099414(n+1)-1. - Robert Price, Nov 09 2014

MATHEMATICA

Do[ If[ PrimeQ[40*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]

PROG

(MAGMA) [n: n in [0..300] | IsPrime((4*10^(n+1)+41)div 9)]; Vincenzo Librandi, Nov 10 2014

CROSSREFS

Sequence in context: A154775 A210418 A167454 * A247963 A105288 A242915

Adjacent sequences:  A056680 A056681 A056682 * A056684 A056685 A056686

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 10 2000

EXTENSIONS

a(8)-a(10) derived from A099414 by Robert Price, Nov 09 2014

STATUS

approved

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Last modified November 26 08:56 EST 2014. Contains 250021 sequences.