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A056657 Numbers n such that 60*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 8
0, 1, 5, 7, 8, 10, 19, 22, 40, 62, 65, 118, 121, 148, 251, 283, 304, 591, 745, 874, 1203, 1363, 2239, 2402, 5105, 5775, 5812, 12455, 14234, 39605, 55543, 84238 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Also numbers n such that (20*10^n+1)/3 is prime.

LINKS

Table of n, a(n) for n=1..32.

Makoto Kamada, Prime numbers of the form 66...667.

Index entries for primes involving repunits

FORMULA

a(n) = A096507(n) - 1.

EXAMPLE

7, 67, 666667, 66666667, 666666667, 66666666667, etc. are primes.

MATHEMATICA

Do[ If[ PrimeQ[ 60*(10^n - 1)/9 + 7 ], Print[n]], {n, 25556}]

CROSSREFS

Cf. A002275, A093170, A096507.

Sequence in context: A314372 A314373 A192270 * A068312 A082097 A088299

Adjacent sequences:  A056654 A056655 A056656 * A056658 A056659 A056660

KEYWORD

nonn

AUTHOR

Robert G. Wilson v, Aug 09 2000

EXTENSIONS

More terms from Robert G. Wilson v, Oct 22 2003

2239,2402,5105,5775 from Farideh Firoozbakht, Dec 23 2003

39605 and 55543 from Serge Batalov, Jun 2009

84238 from Serge Batalov, Jul 06 2009 confirmed as next term by Ray Chandler, Feb 23 2012

STATUS

approved

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Last modified February 21 01:03 EST 2020. Contains 332086 sequences. (Running on oeis4.)