Rost invariant

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In mathematics, the Rost invariant is a cohomological invariant of an absolutely simple simply connected algebraic group G over a field k, which associates an element of the Galois cohomology group H3(k, Q/Z(2)) to a principal homogeneous space for G. Here the coefficient group Q/Z(2) is the tensor product of the group of roots of unity of an algebraic closure of k with itself. MarkusRost  ( 1991 ) first introduced the invariant for groups of type F4 and later extended it to more general groups in unpublished work that was summarized by Serre ( 1995 ).

In mathematics, a cohomological invariant of an algebraic group G over a field is an invariant of forms of G taking values in a Galois cohomology group.

In mathematics, in the field of group theory, a group is said to be absolutely simple if it has no proper nontrivial serial subgroups. That is, is an absolutely simple group if the only serial subgroups of are , and itself.

Algebraic group group that is an algebraic variety

In algebraic geometry, an algebraic group is a group that is an algebraic variety, such that the multiplication and inversion operations are given by regular maps on the variety.

The Rost invariant is a generalization of the Arason invariant.

In mathematics, the Arason invariant is a cohomological invariant associated to a quadratic form of even rank and trivial discriminant and Clifford invariant over a field k of characteristic not 2, taking values in H3(k,Z/2Z). It was introduced by (Arason 1975, Theorem 5.7).

Definition

Suppose that G is an absolutely almost simple simply connected algebraic group over a field k. The Rost invariant associates an element a(P) of the Galois cohomology group H3(k,Q/Z(2)) to a G-torsor P.

The element a(P) is constructed as follows. For any extension K of k there is an exact sequence

where the middle group is the étale cohomology group and Q/Z is the geometric part of the cohomology. Choose a finite extension K of k such that G splits over K and P has a rational point over K. Then the exact sequence splits canonically as a direct sum so the étale cohomology group contains Q/Z canonically. The invariant a(P) is the image of the element 1/[K:k] of Q/Z under the trace map from H3
et
(PK,Q/Z(2)) to H3
et
(P,Q/Z(2)), which lies in the subgroup H3(k,Q/Z(2)).

In mathematics, the étale cohomology groups of an algebraic variety or scheme are algebraic analogues of the usual cohomology groups with finite coefficients of a topological space, introduced by Grothendieck in order to prove the Weil conjectures. Étale cohomology theory can be used to construct ℓ-adic cohomology, which is an example of a Weil cohomology theory in algebraic geometry. This has many applications, such as the proof of the Weil conjectures and the construction of representations of finite groups of Lie type.

These invariants a(P) are functorial in field extensions K of k; in other words the fit together to form an element of the cyclic group Inv3(G,Q/Z(2)) of cohomological invariants of the group G, which consists of morphisms of the functor K→H1(K,G) to the functor K→H3(K,Q/Z(2)). This element of Inv3(G,Q/Z(2)) is a generator of the group and is called the Rost invariant of G.

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References

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