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Primes congruent to 1 or 9 mod 20.
5

%I #16 Sep 08 2022 08:46:03

%S 29,41,61,89,101,109,149,181,229,241,269,281,349,389,401,409,421,449,

%T 461,509,521,541,569,601,641,661,701,709,761,769,809,821,829,881,929,

%U 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

%N Primes congruent to 1 or 9 mod 20.

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

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

%H Vincenzo Librandi, <a href="/A216815/b216815.txt">Table of n, a(n) for n = 1..1000</a>

%t Select[Prime[Range[300]], MemberQ[{1, 9}, Mod[#, 20]]&] (* _Vincenzo Librandi_, Mar 22 2013 *)

%o (Magma) [p: p in PrimesUpTo(2000) | p mod 20 in [1, 9]]; // _Vincenzo Librandi_, Mar 22 2013

%Y A subsequence of A033205. Cf. A216816.

%K nonn,easy

%O 1,1

%A _N. J. A. Sloane_, Sep 20 2012