A141881 Primes congruent to 1 mod 20.

41, 61, 101, 181, 241, 281, 401, 421, 461, 521, 541, 601, 641, 661, 701, 761, 821, 881, 941, 1021, 1061, 1181, 1201, 1301, 1321, 1361, 1381, 1481, 1601, 1621, 1721, 1741, 1801, 1861, 1901, 2081, 2141, 2161, 2221, 2281, 2341, 2381, 2441, 2521, 2621, 2741, 2801

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

Cf. A000040, A105854, A141882, A141883.

Sequence in context: A110411 A145022 A154763 * A139952 A195573 A260554

Adjacent sequences: A141878 A141879 A141880 * A141882 A141883 A141884

Keyword

nonn,easy

Author

N. J. A. Sloane, Jul 11 2008

Status

approved