

A056656


Numbers n such that 20*R_n + 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n.


2



0, 1, 2, 7, 10, 35, 94, 100, 127, 259, 350, 466, 644, 1010, 1177, 1216, 2441, 3760, 3805, 15616, 26458, 63116, 88544, 93496
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

1,3


COMMENTS

Also numbers n such that (2*10^(n+1)+7)/9 is prime.


LINKS

Table of n, a(n) for n=1..24.
Makoto Kamada, Prime numbers of the form 22...223
Index entries for primes involving repunits


FORMULA

a(n) = A096506(n)  1.


MATHEMATICA

Do[ If[ PrimeQ[ 20*(10^n  1)/9 + 3 ], Print[n]], {n, 7000}]


CROSSREFS

Cf. A093162 (corresponding primes).
Sequence in context: A215407 A084184 A015963 * A151346 A247166 A110739
Adjacent sequences: A056653 A056654 A056655 * A056657 A056658 A056659


KEYWORD

nonn


AUTHOR

Robert G. Wilson v, Aug 09 2000


EXTENSIONS

2441 from Rick L. Shepherd, Mar 27 2004
15616 and 26458 from Erik Branger, Jan 31 2010
63116, 88544 and 93496 from Erik Branger, Mar 14 2011; confirmed as next terms by Ray Chandler, Feb 17 2012


STATUS

approved



