

A056678


Numbers n such that 20*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n.


2



1, 2, 4, 13, 175, 415, 2504, 2758, 7924, 9400, 10390, 12104, 19615
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OFFSET

1,2


COMMENTS

Also numbers n such that (2*10^(n+1)+61)/9 is prime.
a(14) > 10^5.  Robert Price, Nov 01 2014


LINKS

Table of n, a(n) for n=1..13.
Makoto Kamada, Prime numbers of the form 22...229.
Index entries for primes involving repunits


FORMULA

a(n) = A099410(n+1)  1.  Robert Price, Nov 01 2014


MATHEMATICA

Do[ If[ PrimeQ[20*(10^n  1)/9 + 9], Print[n]], {n, 5000}]


CROSSREFS

Cf. A002275, A093401 (corresponding primes), A099410.
Sequence in context: A055463 A091957 A327443 * A218655 A325117 A193232
Adjacent sequences: A056675 A056676 A056677 * A056679 A056680 A056681


KEYWORD

hard,nonn


AUTHOR

Robert G. Wilson v, Aug 10 2000


EXTENSIONS

Two more terms from Rick L. Shepherd, Mar 30 2004
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(11)a(13) derived from A099410 by Robert Price, Nov 01 2014


STATUS

approved



