The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A046865 Numbers k such that 4*5^k - 1 is prime. 16
 0, 1, 3, 9, 13, 15, 25, 39, 69, 165, 171, 209, 339, 2033, 6583, 15393, 282989, 498483, 504221, 754611, 864751 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS a(22) > 1000000. - Karsten Bonath, Apr 04 2019 REFERENCES R. K. Guy, Unsolved Problems in Number Theory, Section A3. LINKS Steven Harvey, Williams Primes H. C. Williams, The primality of certain integers of the form 2Ar^n - 1, Acta Arith. 39 (1981), 7-17. MATHEMATICA Print[0]; Do[ If[ PrimeQ[4*5^n - 1], Print[n]], {n, 1, 8100, 2}] PROG (PARI) is(n)=isprime(4*5^n-1) \\ Charles R Greathouse IV, Feb 07 2017 CROSSREFS Cf. A003307, A002235, A079906, A046866, A001771, A005541, A056725, A046867, A079907. Sequence in context: A234286 A193927 A111230 * A063791 A077791 A176393 Adjacent sequences:  A046862 A046863 A046864 * A046866 A046867 A046868 KEYWORD nonn,hard,nice,more AUTHOR EXTENSIONS Two more terms from Robert G. Wilson v, Jan 16 2003 and Jan 26 2003 a(16) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 05 2008 a(1)=0 prepended by Robert Price, Feb 27 2015 a(17) from Karsten Bonath, Dec 07 2018 a(18)-a(19) from Karsten Bonath, Jan 17 2019 a(20)-a(21) from Karsten Bonath, Apr 04 2019 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
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 24 18:33 EDT 2020. Contains 334580 sequences. (Running on oeis4.)