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I have $GF(2^3)$ field defined by $\Pi(x)=x^3+x+1$. From literature they say those are the minimal polynomial, but I can't understand the operative method to find them. Any explanation for a general method?

$$\begin{array}{lll} \textbf{Elem.} & \textbf{Polyn.} & \color{red}{\textbf{Minimal Polyn.}} \\ 0 & 0 & \color{red}{x} \\ \alpha^0 & 1 & \color{red}{x+1} \\ \alpha^1 & \alpha & \color{red}{x^3+x+1} \\ \alpha^2 & \alpha^2 & \color{red}{x^3+x+1} \\ \alpha^3 & \alpha+1 & \color{red}{x^3+x^2+1} \\ \alpha^4 & \alpha^2+\alpha & \color{red}{x^3+x+1} \\ \alpha^5 & \alpha^2+\alpha+1 & \color{red}{x^3 + x^2 + 1} \\ \alpha^6 & \alpha^2+1 & \color{red}{x^3 + x^2 + 1} \\ \end{array}$$

Note: I saw another post about minimal polynomial but there was no such method explained

Alessar
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    For the field of eight elements you can use the fact that $\alpha,\alpha^2$ and $\alpha^4$ are Galois conjugates and hence share a minimal polynomial. The same applies to $\alpha^3,(\alpha^3)^2=\alpha^6$ and $(\alpha^6)^2=\alpha^5$. As $\alpha^6$ is the reciprocal of $\alpha$, the respective minimal polynomials are each others reciprocals as well. Note that $\alpha^5=1/\alpha^2$ and $\alpha^3=1/\alpha^4$ so the reciprocal relation holds for the entire class of conjugates. – Jyrki Lahtonen Dec 21 '18 at 08:16
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    For a method of sorts ... there are several. I outlined one here. A possibly simpler one is described here, but that works only when the element of interest generates the extension field. – Jyrki Lahtonen Dec 21 '18 at 08:20
  • Maybe I miss something, I can't obtain from your example the linear system (cit):

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    At this point we can set up a linear system for the unknowns $c_0,c_1,\cdots,c_5$ and find a solution of the system $$ c_0+c_1\beta+c_2\beta^2+c_3\beta^3+c_4\beta^4+c_5\beta^5=0 $$ by plugging in the values of the powers $\beta^i$ and then set the coefficients of all the powers of $\alpha$ to zero (the powers $1,\alpha,\cdots,\alpha^4$ are linearly independent (because $4<m=5$) so this is valid."

     – Alessar Dec 21 '18 at 12:44
  • @JyrkiLahtonen I understand now. Thank you so much for you great link!!!! – Alessar Dec 21 '18 at 14:53

1 Answers1

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The elements of $GF(8)$ are exactly the roots of the polynomial $X^8-X\in GF(2)[X]$. This polynomial decomposes into irreducible polynomials as follows, $$X^8-X = X(X+1)(X^3+X+1)(X^3+X^2+1).$$ If the field $GF(8)$ is given as $GF(2)[X]/\langle X^3+X+1\rangle$ and $\alpha$ is a zero of $X^3+X+1$, then (as stated above), $\alpha,\alpha^2,\alpha^4$ are the zeros of $X^3+X+1$ and $\alpha^3,(\alpha^3)^2=\alpha^6,((\alpha^3)^2)^2=\alpha^5$ are the zeros of $X^3+X^2+1$.

Wuestenfux
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  • So, knowing the irriducible polynomial over $GF(8)$, I simply try plugging the $\alpha^i$ into one or another and the one who is the zero is it's minimal polynomial? – Alessar Dec 22 '18 at 08:29
  • Indeed, that's correct – Wuestenfux Dec 22 '18 at 08:35
  • And to find the irriducible polynomial over a given field, one must show that $p(0)=p(1)\equiv 1 \pmod p$ and that the polynomial has an odd number of factor, right? Because I think this is the tricky part of finding minimal polynomial – Alessar Dec 22 '18 at 08:39