# Math Help - Algebraic Closure.

1. ## Algebraic Closure.

Does there exists a finite field such is algebraically closed?

I tried of course the most obvious example $\mathbb{Z}_2$ but it is not algebraically closed for $x^2+x+1\in F[\mathbb{Z}_2]$ has no zero.

This problem seems strongly connected with number theory because it invloves prime numbers because all finite fields have order $p^n$.

2. No finite field is algebraically closed. Consider the field of q elements (q some prime power). Then every field element satisfies the equation $X^q = X$, and so the polynomial $X^q - X - 1$ has no roots in the field.

(For simplicity sake I am working with $\mathbb{Z}_p$ only)

1)Every finite field has form $\mathbb{Z}_{p}$.
2)Assume that $\mathbb{Z}_{p}$ is closed.
3)Then $x^2-a\in \mathbb{Z}_{p}[x],\forall a\in\mathbb{Z}_{p}$
4)Thus, $x^2-a\equiv 0 \mod (p)$.
5)Thus, $x^2\equiv a \mod (p)$.
6)Thus, $(a/p)=1$ for all $a\in\mathbb{Z}_p, a\not =0$ (Legendre symbol).
7)Thus, $\sum^{p-1}_{k=1}(k/p)=p-1$.
8)But, $\sum^{p-1}_{k=1}(k/p)=0$ (a theorem)
10)Thus, no finite field of form $\mathbb{Z}_{p}$ is algebraically closed.
Now perhaps it can be completed for all field of $\mathbb{Z}_{p^n}$