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 A216816 Primes congruent to 3 or 7 mod 20. 5

%I

%S 3,7,23,43,47,67,83,103,107,127,163,167,223,227,263,283,307,347,367,

%T 383,443,463,467,487,503,523,547,563,587,607,643,647,683,727,743,787,

%U 823,827,863,883,887,907,947,967,983,1063,1087,1103,1123,1163,1187,1223,1283,1303,1307,1327,1367,1423,1427,1447,1483,1487

%N Primes congruent to 3 or 7 mod 20.

%C This is a subset of A106865 but it is an important sequence in its own right.

%C This may actually be the same sequence as A122870, I did not check. If it is, I will merge the two entries.

%C (a(n) = A122870(n) at least up to n<=8800. _R. J. Mathar_, Sep 25 2012)

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

%H Vincenzo Librandi, <a href="/A216816/b216816.txt">Table of n, a(n) for n = 1..1000</a>

%t Select[Prime[Range[300]], MemberQ[{3, 7}, Mod[#, 20]]&] (* _Vincenzo Librandi_, Jan 06 2013 *)

%o (MAGMA) [p: p in PrimesUpTo(1500) | p mod 20 in [3, 7]]; // _Vincenzo Librandi_, Jan 06 2013

%Y A subset of A106865. Cf. A216815, A122870.

%K nonn,easy

%O 1,1

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

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Last modified October 22 22:34 EDT 2019. Contains 328335 sequences. (Running on oeis4.)