Function: nfgaloisapply
Section: number_fields
C-Name: galoisapply
Prototype: GGG
Help: nfgaloisapply(nf,aut,x): apply the Galois automorphism aut to the object
 x (element or ideal) in the number field nf.
Doc: let $\var{nf}$ be a
 number field as output by \kbd{nfinit}, and let \var{aut} be a \idx{Galois}
 automorphism of $\var{nf}$ expressed by its image on the field generator
 (such automorphisms can be found using \kbd{nfgaloisconj}). The function
 computes the action of the automorphism \var{aut} on the object $x$ in the
 number field; $x$ can be a number field element, or an ideal (possibly
 extended). Because of possible confusion with elements and ideals, other
 vector or matrix arguments are forbidden.
  \bprog
  ? nf = nfinit(x^2+1);
  ? L = nfgaloisconj(nf)
  %2 = [-x, x]~
  ? aut = L[1]; /* the nontrivial automorphism */
  ? nfgaloisapply(nf, aut, x)
  %4 = Mod(-x, x^2 + 1)
  ? P = idealprimedec(nf,5); /* prime ideals above 5 */
  ? nfgaloisapply(nf, aut, P[2]) == P[1]
  %6 = 0 \\ !!!!
  ? idealval(nf, nfgaloisapply(nf, aut, P[2]), P[1])
  %7 = 1
 @eprog\noindent The surprising failure of the equality test (\kbd{\%7}) is
 due to the fact that although the corresponding prime ideals are equal, their
 representations are not. (A prime ideal is specified by a uniformizer, and
 there is no guarantee that applying automorphisms yields the same elements
 as a direct \kbd{idealprimedec} call.)

 The automorphism can also be given as a column vector, representing the
 image of \kbd{Mod(x, nf.pol)} as an algebraic number. This last
 representation is more efficient and should be preferred if a given
 automorphism must be used in many such calls.
 \bprog
  ? nf = nfinit(x^3 - 37*x^2 + 74*x - 37);
  ? aut = nfgaloisconj(nf)[2]; \\ @com an automorphism in basistoalg form
  %2 = -31/11*x^2 + 1109/11*x - 925/11
  ? AUT = nfalgtobasis(nf, aut); \\ @com same in algtobasis form
  %3 = [16, -6, 5]~
  ? v = [1, 2, 3]~; nfgaloisapply(nf, aut, v) == nfgaloisapply(nf, AUT, v)
  %4 = 1 \\ @com same result...
  ? for (i=1,10^5, nfgaloisapply(nf, aut, v))
  time = 463 ms.
  ? for (i=1,10^5, nfgaloisapply(nf, AUT, v))
  time = 343 ms.  \\ @com but the latter is faster
 @eprog
