Hermitian vector bundles and extension groups on arithmetic schemes. I. Geometry of numbers   J.-B. Bost and K. Künnemann       We define and investigate extension groups in the context of Arakelov geometry. In this first part, we focus on the group defined by arithmetic 1-extensions over an arithmetic scheme X, and we especially consider the case when X is the spectrum of the ring of integers of some number field.     BostKuennemann-07-01-11.pdf