

A056660


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


2




OFFSET

1,1


COMMENTS

Also numbers n such that (2*10^(n+1)11)/9 is a prime.


LINKS

Table of n, a(n) for n=1..10.
Makoto Kamada, Prime numbers of the form 22...221.
Index entries for primes involving repunits


FORMULA

a(n) = A084832(n)  1.  Robert Price, Jan 30 2015


MATHEMATICA

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


CROSSREFS

Cf. A002275, A084832, A091189 (corresponding primes).
Sequence in context: A302871 A010913 A142988 * A155610 A001541 A322242
Adjacent sequences: A056657 A056658 A056659 * A056661 A056662 A056663


KEYWORD

nonn


AUTHOR

Robert G. Wilson v, Aug 09 2000


EXTENSIONS

a(8) and a(9) from Rick L. Shepherd, Feb 22 2004
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008


STATUS

approved



