Composita Stability Theorems for Enhanced Koszul Properties in Galois Cohomology
Marina Palaisti
TL;DR
The paper develops an abstract composita stability theorem showing that universal Koszulity, a strong homological regularity property of mod-$p$ Galois cohomology, survives under composita of fields realized as amalgamated pro-$p$ free products with Mayer–Vietoris control. It then applies this framework to Pythagorean fields in the pro-$2$ setting, where maximal pro-$2$ Galois groups decompose as amalgams of Demuškin and free factors, proving that admissible composita preserve quadratic and universally Koszul cohomology. These results yield practical inverse Galois obstructions: finitely generated pro-$p$ groups with non-quadratic or non-universally Koszul cohomology cannot occur as maximal pro-$p$ Galois groups in the constructed families. The work ties into Positselski’s philosophy by showing that enhanced Koszul properties persist through composita, expanding the catalogue of elementary-type fields with robust cohomological regularity and reinforcing stability phenomena in Galois cohomology.
Abstract
We investigate how enhanced Koszul properties of Galois cohomology behave under composita of fields. Given fields $K_1$ and $K_2$ containing $μ_p$, with intersection $k$ and compositum $K = K_1K_2$, we formulate an abstract composita stability theorem: under a pro-$p$ amalgam decomposition $G_K \cong G_{K_1} *_{G_k} G_{K_2}$ of maximal pro-$p$ Galois groups, and natural Mayer-Vietoris compatibility assumptions on the mod-$p$ cohomology rings $H^\bullet(G_{K_1},\mathbb F_p)$, $H^\bullet(G_{K_2},\mathbb F_p)$, and $H^\bullet(G_k,\mathbb F_p)$, the quadratic presentation of $H^\bullet(G_K,\mathbb F_p)$ arises from a fiber-product construction on degree-$1$ generators and quadratic relations. Assuming stability of universal Koszulity under this quadratic gluing, we obtain that universal Koszulity of $H^\bullet(G_{K_1},\mathbb F_p)$ and $H^\bullet(G_{K_2},\mathbb F_p)$ implies universal Koszulity of $H^\bullet(G_K,\mathbb F_p)$. As a concrete application, we prove a composita stability theorem for certain Pythagorean fields whose maximal pro-$2$ Galois groups decompose as free pro-$2$ products of Demuškin groups and free factors. For suitable composita $K = K_1K_2$ of such fields, the mod-$2$ Galois cohomology ring $H^\bullet(G_K(2),\mathbb F_2)$ remains quadratic and universally Koszul. This provides large classes of fields, built from local, global, and Pythagorean base fields by admissible extensions and composita, whose maximal pro-$p$ Galois groups have universally Koszul cohomology, and yields inverse Galois obstructions: any finitely generated pro-$p$ group with nonquadratic or non-universally Koszul mod-$p$ cohomology cannot occur as the maximal pro-$p$ Galois group of a field in these families.