This site is supported by donations to The OEIS Foundation.

Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 5000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A099419 Numbers n such that 7*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3
 2, 13, 20, 23, 31, 100, 241, 275, 925, 1067, 1369, 2065, 7163, 37963, 91856, 111706, 260198, 271757, 314564, 348724 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also numbers n such that (7*10^n-61)/9 is prime. LINKS Makoto Kamada, Prime numbers of the form 77...771. FORMULA a(n) = A056688(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 MATHEMATICA Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 6], Print[n]], {n, 0, 7000}] CROSSREFS Cf. A002275, A056688, A093176. Sequence in context: A007355 A105452 A219278 * A061871 A182459 A084651 Adjacent sequences:  A099416 A099417 A099418 * A099420 A099421 A099422 KEYWORD nonn AUTHOR Robert G. Wilson v, Oct 14 2004 EXTENSIONS a(13) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 a(14)-a(15) from Erik Branger, Dec 19 2009 a(16) from Erik Branger, Jan 29 2011 a(17) from Erik Branger, Nov 26 2011 a(18) from Erik Branger, Mar 05 2012 a(19)-a(20) from Erik Branger, Sep 23 2016 STATUS approved

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