Cremona group
（重定向自Cremona transformation）
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(P(k)) or Bir(P(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.