The Hasse principle for bilinear symmetric forms over the ring of integers of a global function field
יום ד', 29/04/2015 - 10:30
Dr. Rony Bitan (Bar-Ilan University)
Third floor seminar room
Let C be a smooth projective curve defined over a finite field F_q (q is odd), and let K=F_q(C) be its (global) function field. This field is considered as the geometric analogue of a number field.
Removing one closed point from C results in an affine curve C^af. The ring of regular functions over C^af is an integral domain, over which we consider a non-degenerate bilinear and symmetric form f of any rank n.
We express the number c(f) of isomorphism classes in the genus of f in cohomological terms and use it to present a sufficient and necessary condition depending only on C^af, under which f admits the Hasse local-global principle. We say that f admits the Hasse local-global principle if c(f)=1, namely, |Pic(C^\af)| is odd for any n other than 2 and equal to 1 for n=2.
This result emphasizes the difference between Galois cohomology and etale cohomology. Examples are provided.