login
This site is supported by donations to The OEIS Foundation.

 

Logo

Invitation: celebrating 50 years of OEIS, 250000 sequences, and Sloane's 75th, there will be a conference at DIMACS, Rutgers, Oct 9-10 2014.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A056700 Numbers n such that 2*10^n + R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
0, 2, 3, 12, 18, 23, 57, 128, 543, 584, 833, 2450, 2810, 2873, 3671, 6384, 10296, 16704, 53049, 56544, 74253 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that (19*10^n-1)/9 is prime.

LINKS

Table of n, a(n) for n=1..21.

Makoto Kamada, Factorizations of 211...11

Index entries for primes involving repunits

MATHEMATICA

Do[ If[ PrimeQ[ 2*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]

CROSSREFS

Cf. A068814.

Sequence in context: A073452 A112976 A100570 * A140989 A130089 A126292

Adjacent sequences:  A056697 A056698 A056699 * A056701 A056702 A056703

KEYWORD

nonn

AUTHOR

Robert G. Wilson v, Aug 10 2000

EXTENSIONS

6384 from Hugo Pfoertner, Oct 16 2004

10296 and 16704 from Ray Chandler, Sep 2010

0 added by Arkadiusz Wesolowski, Mar 10 2011

53049 and 56544 from Serge Batalov, Mar 2009 confirmed as next terms by Ray Chandler, Feb 03 2012

74253 from Ray Chandler, Feb 03 2012

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement .

Last modified July 28 10:37 EDT 2014. Contains 244989 sequences.