A septic covariant and the Hermite--Joubert problem in degree seven
We prove the degree-seven case of the Hermite--Joubert problem in characteristic zero: if $F$ is a field of characteristic zero and $E/F$ is a field extension of degree seven, then $E$ is generated by an element $a$ with $\text{tr}_{E/F}(a)=\text{tr}_{E/F}(a^{3})=0$, that is, with minimal polynomial of the form $λ^{7}+c_{2}λ^{5}+c_{4}λ^{3}+c_{5}λ^{2}+c_{6}λ+c_{7}$, where the $c_i$'s belong to $F$. This is given by an explicit formula: a covariant of the binary septic of coefficient degree seven and order five, evaluated at a generator $θ$ and divided by the derivative of its minimal polynomial evaluated at $θ$. We also announce the general theorem, which will be proved in a companion paper in preparation: over every infinite field, in every characteristic, every étale algebra of degree seven contains a primitive element $a$ with $c_{1}(a)=c_{3}(a)=0$. Moreover, every field extension of degree seven of an arbitrary field has a generator $a$ with $c_{1}(a)=c_{3}(a)=0$.