A040987 - OEIS

%I #15 Jun 19 2023 04:21:03

%S 31,41,71,101,131,151,181,191,211,241,251,271,281,311,331,401,431,491,

%T 521,541,571,701,751,761,811,821,881,911,941,971,991,1021,1031,1051,

%U 1061,1091,1151,1181,1201,1291,1301,1321,1361,1381,1471

%N Primes p such that x^5 = 21 has no solution mod p.

%C Complement of A040986 relative to A000040. - _Vincenzo Librandi_, Sep 19 2012

%H Vincenzo Librandi, <a href="/A040987/b040987.txt">Table of n, a(n) for n = 1..1000</a>

%t ok[p_] := Reduce[Mod[x^5 - 21, p] == 0, x, Integers] == False; Select[Prime[Range[300]], ok] (* _Vincenzo Librandi_, Sep 19 2012 *)

%o (Magma) [p: p in PrimesUpTo(1500) | not exists{x: x in ResidueClassRing(p) | x^5 eq 21}]; // _Vincenzo Librandi_, Sep 19 2012

%K nonn,easy

%O 1,1

%A _N. J. A. Sloane_