This site is supported by donations to The OEIS Foundation.

 Annual appeal: Please make a donation to keep the OEIS running! Over 6000 articles have referenced us, often saying "we discovered this result with the help of the OEIS". Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A007647 Numbers n such that n*10^n + 1 is prime. (Formerly M2783) 5
 1, 3, 9, 21, 363, 2161, 4839, 49521, 105994, 207777 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS These are generalized Cullen numbers in base 10. - Julien Peter Benney (jpbenney(AT)ftml.net), Oct 24 2004 No others less than 270026. - Ray Chandler, Apr 10 2016 REFERENCES J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 363, p. 84, Ellipses, Paris 2008. H. Dubner, Generalized Cullen numbers, J. Rec. Math., 21 (No. 3, 1989), 190-191. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Daniel Hermle, First Coordinated Generalized Cullen Prime Search Guenter Loeh, Generalized Cullen primes. EXAMPLE For n = 3 we get (3*10^3)+1 = (3*1000)+1 = 3000 + 1 = 3001, which is prime. For n = 9 we get 9*10^9+1 = 9*1000000000+1 = 9000000000+1 = 9000000001, which is prime. PROG (PARI) is(n)=ispseudoprime(n*10^n+1) \\ Charles R Greathouse IV, Feb 17 2017 CROSSREFS Cf. A004023. Sequence in context: A193374 A191998 A098980 * A247182 A063586 A131477 Adjacent sequences:  A007644 A007645 A007646 * A007648 A007649 A007650 KEYWORD hard,nonn AUTHOR EXTENSIONS More terms from Julien Peter Benney (jpbenney(AT)ftml.net), Jun 11 2005 Edited by N. J. A. Sloane at the suggestion of Andrew Plewe, Jun 05 2007 a(10) from Loeh link by Ray Chandler, Apr 10 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.