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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

a(20) > 695000. - Karsten Bonath, Jan 17 2019

REFERENCES

R. K. Guy, Unsolved Problems in Number Theory, Section A3.

LINKS

Table of n, a(n) for n=1..19.

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,changed

AUTHOR

N. J. A. Sloane

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

STATUS

approved

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Last modified January 20 12:29 EST 2019. Contains 319330 sequences. (Running on oeis4.)