The existence of small defining polynomials for number fields, II

Abstract

We prove that every number field of odd degree $n\geq 3$ and absolute discriminant $D$ has an integral defining polynomial whose coefficients are $O_n(D^{(n-1)/(2n-1)})$ in size. This strengthens, in odd-degree cases, the recently established bound of $O_n(D^{1/2})$ for all degrees $n$, which was conjectured in 1998 by Ruppert and is now known to be optimal in even degrees. More generally, we prove that this strengthened bound holds for all degree-$n$ fields having a real embedding and no subfield of index two. The proof makes use of the lattice shapes of the ring of integers and its trace dual, the inverse different. Assuming the quasi-Riemann hypothesis for Dedekind zeta functions, as recently announced by Cureton and Thorner, we obtain an exponent smaller than $1/2$ for every field with no index-two subfield, of any signature. In the case of primitive fields, we obtain the exponent $(n-1)/(2n)$. In particular, we prove that the optimal exponent for cubic fields is $1/3$.