Field F_9 - Knowlpedia

The field $\mathbb F_9$ is the finite field presented by

\mathbb F_9=\mathbb F_3[x]/(x^2+1).

Writing $\alpha$ for the residue class of $x$ , every element has a unique representative of degree less than $2$ , and $\alpha^2=-1$ .

Why this is a field

The polynomial $x^2+1$ has no root in $\mathbb F_3$ . Since its degree is two, it is irreducible; the quotient is therefore a field. Counting coefficients gives exactly 9 elements.

Field maps versus linear maps

The field automorphism group has order $2$ , generated by $a\mapsto a^3$ . The additive space has dimension $2$ over $\mathbb F_3$ , so its linear automorphism group is $\mathrm{GL}_2(\mathbb F_3)$ . A general invertible linear map need not preserve products or the unit.

Recorded linear maps

Its linear endomorphism ring over $\mathbb F_3$ is $M_2(\mathbb F_3)$ , with 81 maps. Its invertible linear maps form $\mathrm{GL}_2(\mathbb F_3)$ , with 48 elements. The catalogue keeps this scalar category separate from field morphisms, for which only the 2 Frobenius powers occur.

References

J. S. Milne, Fields and Galois Theory, Chapter 4, Propositions 4.19–4.23, pp. 53–54.