OFFSET
1,1
COMMENTS
Such a prime is representable by either both or neither of the quadratic forms x^2 + 20 y^2 and x^2 + 100 y^2. See the Brink link. - Robert Israel, Jun 11 2014
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
David Brink, Five peculiar theorems on simultaneous representation of primes by quadratic forms, Journal of Number Theory 129(2) (2009), 464-468, doi:10.1016/j.jnt.2008.04.007, MR 2473893
MAPLE
select(isprime, [seq(20*i+1, i=1..1000)]); # Robert Israel, Jun 11 2014
MATHEMATICA
Select[Range[1, 5000, 20], PrimeQ[#]&] (* Vladimir Joseph Stephan Orlovsky, Mar 31 2011*)
PROG
(Magma) [p: p in PrimesUpTo(3000) | p mod 20 eq 1 ]; // Vincenzo Librandi, Aug 15 2012
(PARI) is(n)=isprime(n) && n%20==1 \\ Charles R Greathouse IV, Jul 01 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, Jul 11 2008
STATUS
approved