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
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