

A056655


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


2



0, 1, 3, 4, 7, 22, 28, 39, 130, 135, 214, 610, 766, 2152, 2575, 22972, 42688, 85711, 85863
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OFFSET

1,3


COMMENTS

Also numbers n such that (10^(n+1)+53)/9 is prime.
2575 also produces a probable prime.
a(20) > 10^5.  Robert Price, Jan 13 2015


LINKS

Table of n, a(n) for n=1..19.
Makoto Kamada, Prime numbers of the form 11...117.
Index entries for primes involving repunits


FORMULA

a(n) = A097684(n)  1 for all n >= 0.  Rick L. Shepherd, Aug 23 2004


MATHEMATICA

Do[ m = n; If[ primeQ[ 10*(10^n  1)/9 + 7 ], Print[ n ] ], {n, 1, 1250} ]


CROSSREFS

Cf. A093139 (corresponding primes), A097684.
Sequence in context: A241660 A030724 A124082 * A332971 A292033 A288501
Adjacent sequences: A056652 A056653 A056654 * A056656 A056657 A056658


KEYWORD

hard,nonn


AUTHOR

Robert G. Wilson v, Aug 09 2000


EXTENSIONS

2152 (giving a probable prime) from Rick L. Shepherd, Mar 23 2004
2575 from Rick L. Shepherd, Aug 23 2004
a(16)a(19) derived from A097684 by Robert Price, Jan 13 2015


STATUS

approved



