Linear Relations Among Galois Conjugates Over ${\mathbb{F}}_q(t)$

Abstract

We classify the coefficients $(a_1,...,a_n)\in {\mathbb{F}}_q[t{]}^n$ that can appear in a linear relation ${\sum }_{i=1}^n{a}_i{\gamma }_i=0$ among Galois conjugates ${\gamma }_i\in \overline{{\mathbb{F}}_q(t)}$. We call such an $n$-tuple a Smyth tuple. Our main theorem gives an affirmative answer to a function field analogue of a 1986 conjecture of Smyth over $\mathbb{Q}$. Smyth showed that certain local conditions on the $a_i$ are necessary and conjectured that they are sufficient. Our main result is that the analogous conditions are necessary and sufficient over ${\mathbb{F}}_q(t)$, which we show using a combinatorial characterization of Smyth tuples due to Smyth. We also formulate a generalization of Smyth's Conjecture in an arbitrary number field that is not a straightforward generalization of the conjecture over $\mathbb{Q}$ due to a subtlety occurring at the archimedean places.

16 pages