I forget to add few more equations to system ie. {x^4-x,y^4-y,z^4-z}
in reason to have solution in GF(4) and zero dimensional ideal, so I
answered my question.
But, I have another issue.
When I use quotient ring Q, J.groebner_basis() does not return
completely reduced basis.
Also, with quotient ring Q, J.variety() returns ValueError: Could not
detect ring.

Code:
K.<alpha>=GF(4)
R.<a,b,c>=PolynomialRing(K,3,order='lex')
Q.<x,y,z>=R.quotient(sage.rings.ideal.FieldIdeal(R))
R.inject_variables()
Q.inject_variables()
F=[a^3+a+c,b+c,a^4-a,b^4-b,c^4-c]
G=[x^3+x+z,y+z]
I=Ideal(F)
J=Ideal(G)
print I.groebner_basis()
print J.groebner_basis()
   Defining a, b, c
   Defining x, y, z
   [a^2 + a + c^2 + c, a*c + c^2 + c, b + c, c^3 + c^2 + c]
   [x^2 + x*z + x, x*z + z^2 + z, y + z, z^3 + z^2 + z]
J.variety()
   Traceback (click to the left of this block for traceback)
   ...
   ValueError: Could not detect ring.

On Oct 26, 5:58 pm, Dušan Orlović <duleorlo...@gmail.com> wrote:
> Hi all,
> I have a question how to solve system in GF(4) using Grobner basis and
> elimination (I want to find just one solution).
> The system (which is already in reduced form) is:
>
> x^3+x+z
> y+z
>
> If we put z=1 then first equation x^3+x+1 has no root in GF(4). If z=0 we
> have solution y=0, x=0.
> Is there an algorithm that can determine right values of z instead of trying
> all values?
> Thanks,

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