

A056693


Numbers n such that 70*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n.


2




OFFSET

1,2


COMMENTS

Also numbers n such that (7*10^(n+1)+11)/9 is prime.


LINKS

Table of n, a(n) for n=1..9.
Makoto Kamada, Prime numbers of the form 77...779.
Index entries for primes involving repunits


FORMULA

a(n) = A098089(n+1)  1.  Robert Price, Nov 22 2014


MATHEMATICA

Do[ If[ PrimeQ[70*(10^n  1)/9 + 9], Print[n]], {n, 0, 5000}]
Select[Range[700], PrimeQ[(7 10^(# + 1) + 11) / 9] &] (* Vincenzo Librandi, Nov 22 2014 *)


PROG

(MAGMA) [n: n in [1..400]  IsPrime((7*10^(n+1)+11) div 9)]; Vincenzo Librandi, Nov 22 2014


CROSSREFS

Cf. A002275, A093404, A098089.
Sequence in context: A113688 A214484 A159758 * A164282 A025312 A024508
Adjacent sequences: A056690 A056691 A056692 * A056694 A056695 A056696


KEYWORD

hard,nonn


AUTHOR

Robert G. Wilson v, Aug 10 2000


EXTENSIONS

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(9) derived from A098089 by Robert Price, Nov 22 2014


STATUS

approved



