Quotients d'une variété algébrique par un groupe algébrique linéairement
réductif et ses
sous-groupes maximaux unipotents

The topological notion of a quotient is fairly simple. Given a topological group $G$ acting on a topological space $X$, one gets the natural application from $X$ to the quotient space $X/G$. In algebraic geometry, unfortunately, it is generally not possible  to give the orbit space the structure of an algebraic variety. In the special case of a linearly reductive group acting on a projective variety $X$, the geometric invariant theory allows us to get a morphism of variety from an open $U$ of $X$ to a projective variety $X//G$, which is as close as possible to a quotient map, from a topological point of view.  As an example, let $ X\subseteq P^{n}$ be a $k$-projective variety on which acts a linearly reductive group $G$. Suppose further that this action is induced by a linear action of $G$ on $A^{n+1}$ and let $\widehat{X}\subseteq A^{n +1}$ be the affine cone over $X$. By an important theorem of the classical invariants theory, there exist homogeneous invariants $f_{1},..., f_{r}\in C[\widehat{X}]^{G}$ such as $$\C[\widehat{X}]^{G}=\C[f_{1},...,f_{r}].$$ The locus in $X$ of $f_{1},...,f_{r}$ is called the nullcone, noted $N$. Let $Proj(C[\widehat{X}]^{G})$ be the projective spectrum of the invariants ring. The rational map $$\pi:X\dashrightarrow Proj(C[f_{1},...,f_{r}])$$ induced by the inclusion of $C[\widehat{X}]^{G}$ in $C[\widehat{X}] $ is then surjective, constant on the orbits and separates orbits as much as possible, that is, the fibres contains exactly one closed orbit. A regular map is obtained by removing the nullcone; we then get a regular map $$\pi:X \backslash N\rightarrow Proj(C[f_{1},...,f_{r}])$$ which still satisfy the preceding properties. The Hilbert-Mumford criterion, due to Hilbert and revisited by Mumford nearly half-century later, can be used to describe $N$ without knowing the generators of the invariants ring. Since those are rarely known, this criterion had proved to be quite useful. Despite the important applications of this criterion in classical algebraic geometry, the demonstrations found in the literature are usually given trough the difficult theory of schemes. The aim of this master thesis is therefore, among others, to provide a demonstration of this criterion using classical algebraic geometry and of commutative algebra. The version that we demonstrate is somewhat wider than the original version of Hilbert \cite{hilbert}; a schematic proof of this general version is given in \cite{kempf}. Finally, the proof given here is valid for $C$ but could be generalised to a field $k$ of characteristic zero, not necessarily algebraically closed.  In the second part of this thesis, we study the relationship between the preceding constructions and those obtained by including covariants in addition to the invariants. We give a Hilbert-Mumford criterion for covariants (Theorem 6.3.2) which is a theorem from Brion for which we prove a slightly more general version. This theorem, together with a simplified proof of a theorem of Grosshans (Theorem 6.1.7), are the elements of this thesis that can't be found in the literature.

Keyphrases: Invariants, Covariants, Quotients, Linearly reductive groups, Maximal unipotent subgroups, 1 parametre subgroups, Invariants, Covariants, Quotients, Hilbert-Mumford criterion, Nullcone