The Segre map may be defined as the map
taking a pair of points to their product
(the XiYj are taken in lexicographical order).
is that of homogeneous coordinates on the space. The image of the map is a variety, called a Segre variety. It is sometimes written as .
In general, this need not be injective because, for in , in and any nonzero in ,
Considering the underlying projective spaces P(U) and P(V), this mapping becomes a morphism of varieties
This is not only injective in the set-theoretic sense: it is a closed immersion in the sense of algebraic geometry. That is, one can give a set of equations for the image. Except for notational trouble, it is easy to say what such equations are: they express two ways of factoring products of coordinates from the tensor product, obtained in two different ways as something from U times something from V.
This mapping or morphism σ is the Segre embedding. Counting dimensions, it shows how the product of projective spaces of dimensions m and n embeds in dimension
Classical terminology calls the coordinates on the product multihomogeneous, and the product generalised to k factors k-way projective space.
Here, is understood to be the natural coordinate on the image of the Segre map.
The Segre variety is the categorical product of and . The projection
to the first factor can be specified by m+1 maps on open subsets covering the Segre variety, which agree on intersections of the subsets. For fixed , the map is given by sending to . The equations ensure that these maps agree with each other, because if we have .
The fibers of the product are linear subspaces. That is, let
be the projection to the first factor; and likewise for the second factor. Then the image of the map
for a fixed point p is a linear subspace of the codomain.
For example with m = n = 1 we get an embedding of the product of the projective line with itself in P3. The image is a quadric, and is easily seen to contain two one-parameter families of lines. Over the complex numbers this is a quite general non-singular quadric. Letting
is known as the Segre threefold. It is an example of a rational normal scroll. The intersection of the Segre threefold and a three-plane is a twisted cubic curve.
The image of the diagonal under the Segre map is the Veronese variety of degree two
Because the Segre map is to the categorical product of projective spaces, it is a natural mapping for describing entangled states in quantum mechanics and quantum information theory. More precisely, the Segre map describes how to take products of projective Hilbert spaces.
In algebraic statistics, Segre varieties correspond to independence models.
The Segre embedding of P2×P2 in P8 is the only Severi variety of dimension 4.
- McKernan, James (2010). "Algebraic Geometry Course, Lecture 6: Products and fibre products". online course material. Retrieved 11 April 2014.