# Algebraic group of transformations

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An algebraic group acting regularly on an algebraic variety . More precisely, it is a triplet where () is a morphism of algebraic varieties satisfying the conditions: , for all and (where is the unit of ). If and are defined over a field , then is called an algebraic group of -transformations. For instance, , where is the adjoint action or an action by shifts, is an algebraic group of transformations. If is an algebraic subgroup in and is its natural action on the affine space , then is an algebraic group of transformations. For each point one denotes by the orbit of , and by the stabilizer of . The orbit need not necessarily be closed in , but closed orbits exist always, e.g. orbits of minimal dimension are closed. An algebraic group of transformations is sometimes understood to mean a group which is acting rationally (but not necessarily regularly) on an algebraic variety (this means that is a rational mapping, and the above properties of are valid for ordinary points). It was shown by A. Weil [3] that there always exists a variety , birationally isomorphic to , and such that the action of on induced by the rational action of on is regular. The problem of describing the orbits, stabilizers, fields of invariant rational functions (cf. Invariants, theory of), and of constructing quotient varieties are fundamental in the theory of algebraic groups of transformations and have numerous applications.

#### References

 [1] A. Borel, "Linear algebraic groups" , Benjamin (1969) [2a] J.A. Dieudonné, "Invariant theory: old and new" , Acad. Press (1971) [2b] D. Mumford, "Geometric invariant theory" , Springer (1965) [2c] D. Mumford, "Projective invariants of projective structures and applications" , Proc. Internat. Congress mathematicians (Stockholm, 1962) , Inst. Mittag-Leffler (1963) pp. 526–530 [2d] C.S. Seshadri, "Quotient spaces modulo reductive groupes and applications to moduli of vector bundles on algebraic curves" , Proc. Internat. Congress Mathematicians (Nice, 1970) , 1 , Gauthier-Villars (1971) pp. 479–482 [3] A. Weil, "On algebraic groups and homogeneous spaces" Amer. J. Math. , 77 : 2 (1955) pp. 355–391