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In algebraic geometry, the Cremona group, introduced by Cremona (1863, 1865), is the group of birational automorphisms of the n-dimensional projective space over a field k. It is denoted by Cr(Pn(k)) or Bir(Pn(k)) or Crn(k).

The Cremona group is naturally identified with the automorphism group Autk(k(x1, ..., xn)) of the field of the rational functions in n indeterminates over k, or in other words a pure transcendental extension of k, with transcendence degree n.

The projective general linear group of order n+1, of projective transformations, is contained in the Cremona group of order n. The two are equal only when n=0 or n=1, in which case both the numerator and the denominator of a transformation must be linear.

The Cremona group in 2 dimensions

In two dimensions, Max Noether and Castelnuovo showed that the complex Cremona group is generated by the standard quadratic transformation, along with PGL(3, k), though there was some controversy about whether their proofs were correct, and Gizatullin (1983) gave a complete set of relations for these generators. The structure of this group is still not well understood, though there has been a lot of work on finding elements or subgroups of it.

Cantat & Lamy (2010) showed that the Cremona group is not simple as an abstract group;
Blanc showed that it has no nontrivial normal subgroups that are also closed in a natural topology.
For the finite subgroups of the Cremona group see Dolgachev & Iskovskikh (2009).

The Cremona group in higher dimensions

There is little known about the structure of the Cremona group in three dimensions and higher though many elements of it have been described. Blanc (2010) showed that it is (linearly) connected, answering a question of Serre (2010). There is no easy analogue of the Noether–Castelnouvo theorem as Hudson (1927) showed that the Cremona group in dimension at least 3 is not generated by its elements of degree bounded by any fixed integer.

De Jonquières groups

A De Jonquières group is a subgroup of a Cremona group of the following form. Pick a transcendence basis x1, ..., xn for a field extension of k. Then a De Jonquières group is the subgroup of automorphisms of k(x1, ..., xn) mapping the subfield k(x1, ..., xr) into itself for some rn. It has a normal subgroup given by the Cremona group of automorphisms of k(x1, ..., xn) over the field k(x1, ..., xr), and the quotient group is the Cremona group of k(x1, ..., xr) over the field k. It can also be regarded as the group of birational automorphisms of the fiber bundle Pr×PnrPr.

When n=2 and r=1 the De Jonquières group is the group of Cremona transformations fixing a pencil of lines through a given point, and is the semidirect product of PGL2(k) and PGL2(k(t)).

When n=2 and r=1 the De Jonquières group is the group of Cremona transformations fixing a pencil of lines through a given point, and is the semidirect product of PGL2(k) and PGL2(k(t)).

References

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Blanc, Jérémy (2010), "Groupes de Cremona, connexité et simplicité", Annales Scientifiques de l'École Normale Supérieure. Quatrième Série 43 (2): 357–364, ISSN 0012-9593, MR 2662668
Cantat, Serge; Lamy, Stéphane (2010). "Normal subgroups in the Cremona group". arXiv:1007.0895.
Coolidge, Julian Lowell (1931), A treatise on algebraic plane curves, Oxford University Press, ISBN 978-0-486-49576-7, MR 0120551
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Dolgachev, Igor V. (2012), Classical Algebraic Geometry: a modern view (PDF), Cambridge University Press, ISBN 978-1-107-01765-8
Dolgachev, Igor V.; Iskovskikh, Vasily A. (2009), "Finite subgroups of the plane Cremona group", Algebra, arithmetic, and geometry: in honor of Yu. I. Manin. Vol. I, Progr. Math. 269, Boston, MA: Birkhäuser Boston, pp. 443–548, doi:10.1007/978-0-8176-4745-2_11, MR 2641179
Gizatullin, M. Kh. (1983), "Defining relations for the Cremona group of the plane", athematics of the USSR-Izvestiya 21 (2): 211–268, doi:10.1070/IM1983v021n02ABEH001789, ISSN 0373-2436, MR 675525
Godeaux, Lucien (1927), Les transformations birationelles du plan, Mémorial des sciences mathématiques 22, Gauthier-Villars et Cie, JFM 53.0595.02
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Hazewinkel, Michiel, ed. (2001), "Cremona_transformation", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
Hudson, Hilda Phoebe (1927), Cremona transformations in plane and space, Cambridge University Press, ISBN 978-0-521-35882-8, Reprinted 2012
Semple, J. G.; Roth, L. (1985), Introduction to algebraic geometry, Oxford Science Publications, The Clarendon Press Oxford University Press, ISBN 978-0-19-853363-4, MR 814690
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Serre, Jean-Pierre (2010), "Le groupe de Cremona et ses sous-groupes finis" (PDF), Astérisque, Seminaire Bourbaki 1000 (332): 75–100, ISBN 978-2-85629-291-4, ISSN 0303-1179, MR 2648675

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