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A056656 Numbers n such that 20*R_n + 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
0, 1, 2, 7, 10, 35, 94, 100, 127, 259, 350, 466, 644, 1010, 1177, 1216, 2441, 3760, 3805, 15616, 26458, 63116, 88544, 93496 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Also numbers n such that (2*10^(n+1)+7)/9 is prime.

LINKS

Table of n, a(n) for n=1..24.

Makoto Kamada, Prime numbers of the form 22...223

Index entries for primes involving repunits

FORMULA

a(n) = A096506(n) - 1.

MATHEMATICA

Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 3 ], Print[n]], {n, 7000}]

CROSSREFS

Cf. A093162 (corresponding primes).

Sequence in context: A215407 A084184 A015963 * A151346 A110739 A179117

Adjacent sequences:  A056653 A056654 A056655 * A056657 A056658 A056659

KEYWORD

nonn

AUTHOR

Robert G. Wilson v, Aug 09 2000

EXTENSIONS

2441 from Rick L. Shepherd, Mar 27 2004

15616 and 26458 from Erik Branger, Jan 31 2010

63116, 88544 and 93496 from Erik Branger, Mar 14 2011; confirmed as next terms by Ray Chandler, Feb 17 2012

STATUS

approved

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Last modified August 22 18:42 EDT 2014. Contains 245976 sequences.