A principal -object in the category of algebraic varieties or schemes. If is a scheme and is a group scheme over , then a principal -object in the category of schemes over is said to be a principal homogeneous space. If is the spectrum of a field (cf. Spectrum of a ring) and is an algebraic -group (cf. Algebraic group), then a principal homogeneous space over is an algebraic -variety acted upon (from the left) by such that if is replaced by its separable algebraic closure , then each point defines an isomorphic mapping of the varieties and . A principal homogeneous space is trivial if and only if is non-empty. The set of classes of isomorphic principal homogeneous spaces over a smooth algebraic group can be identified with the set of Galois cohomology . In the general case the set of classes of principal homogeneous spaces over an -group scheme coincides with the set of one-dimensional non-Abelian cohomology . Here is some Grothendieck topology on the scheme .
Principal homogeneous spaces have been computed in a number of cases. If is a finite field, then each principal homogeneous space over a connected algebraic -group is trivial (Lang's theorem). This theorem also holds if is a -adic number field and is a simply-connected semi-simple group (Kneser's theorem). If is a multiplicative -group scheme, then the set of classes of principal homogeneous spaces over becomes identical with the Picard group of . In particular, if is the spectrum of a field, this group is trivial. If is an additive -group scheme, then the set of classes of principal homogeneous spaces over becomes identical with the one-dimensional cohomology group of the structure sheaf of . In particular, this set is trivial if is an affine scheme. If is a global field (i.e. an algebraic number field or a field of algebraic functions in one variable), then the study of the set of classes of principal homogeneous spaces over an algebraic -group is based on the study of the Tate–Shafarevich set , which consists of the principal homogeneous spaces over with rational points in all completions with respect to the valuations of . If is an Abelian group over the field , then the set of classes of principal homogeneous spaces over forms a group (cf. Weil–Châtelet group).