Explicit Jordan decompositions for ideal lattices in CM fields

Abstract

Let $E$ be a CM number field, $F$ a totally real subfield (e.g. $\mathbb Q$), and let $\mathfrak a$ be a non-zero fractional ideal of $E$. Endowed with the Hermitian trace form $h_{E/F}(x,y)=\mathrm{Tr}_{E/F}(x\overline{y})$, the ideal $\mathfrak a$ defines an ideal lattice over $\mathcal O_F$. In this paper, we give explicit formulas for the Jordan decomposition of this lattice at a prime ideal $\mathfrak p\subset\mathcal O_F$, in terms of the prime ideal factorization of $\mathfrak a$ in $E$. Following the approach of Erez, Morales, and Perlis, we reduce the computation to the local behavior at the primes above $\mathfrak p$. Our results provide local invariants for the isometry relation between ideal lattices, with potential applications to the study of structured lattices arising in arithmetic and cryptography.