

A056664


Numbers n such that 80*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.


3



2, 18, 78, 138, 222, 462, 543, 1095, 1418, 3246, 3876, 4416, 9506, 11090, 14601, 27810, 29187
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OFFSET

1,1


COMMENTS

Also numbers n such that (8*10^(n+1)71)/9 is prime.
a(18) > 10^5.  Robert Price, Nov 01 2014


LINKS

Table of n, a(n) for n=1..17.
Makoto Kamada, Prime numbers of the form 88...881.
Index entries for primes involving repunits


FORMULA

a(n) = A099421(n+1)  1.  Robert Price, Nov 01 2014


MATHEMATICA

Do[ If[ PrimeQ[ 80*(10^n  1)/9 + 1 ], Print[n]], {n, 15000}]


CROSSREFS

Cf. A002275, A092675 (corresponding primes), A099421.
Sequence in context: A197093 A034473 A024171 * A213830 A064057 A176496
Adjacent sequences: A056661 A056662 A056663 * A056665 A056666 A056667


KEYWORD

hard,nonn


AUTHOR

Robert G. Wilson v, Aug 09 2000


EXTENSIONS

1415 (giving a probable prime) from Rick L. Shepherd, Mar 02 2004. There are no other terms <= 2500.
a(16)a(17) derived from A099421 by Robert Price, Nov 01 2014


STATUS

approved



