A216815 Primes congruent to 1 or 9 mod 20.

29, 41, 61, 89, 101, 109, 149, 181, 229, 241, 269, 281, 349, 389, 401, 409, 421, 449, 461, 509, 521, 541, 569, 601, 641, 661, 701, 709, 761, 769, 809, 821, 829, 881, 929, 941, 1009, 1021, 1049, 1061, 1069, 1109, 1129, 1181, 1201, 1229, 1249, 1289, 1301, 1321, 1361, 1381, 1409, 1429, 1481, 1489, 1549, 1601, 1609, 1621, 1669, 1709, 1721, 1741, 1789, 1801, 1861

Offset: 1

Author

N. J. A. Sloane, Sep 20 2012

Keywords

• nonn
• easy

Comments

This is a subsequence of A033205 but it is an important sequence in its own right.

References

• David A. Cox, "Primes of the Form x^2 + n y^2", Wiley, 1989; see p. 33.

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Crossrefs

A subsequence of A033205. Cf. A122870.

Programs

Magma

[p: p in PrimesUpTo(2000) | p mod 20 in [1, 9]]; // Vincenzo Librandi, Mar 22 2013

Mathematica

Select[Prime[Range[300]], MemberQ[{1, 9}, Mod[#, 20]]&] (* Vincenzo Librandi, Mar 22 2013 *)