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In topology, a map is a branched covering if it is a covering map everywhere except for a nowhere dense set known as the branch set. Examples include the map from a wedge of circles to a single circle, where the map is a homeomorphism on each circle.
In algebraic geometry, the term branched covering is used to describe morphisms from an algebraic variety to another one , the two dimensions being the same, and the typical fibre of being of dimension 0.
In that case, there will be an open set of (for the Zariski topology) that is dense in , such that the restriction of to (from to , that is) is unramified.[clarification needed] Depending on the context, we can take this as local homeomorphism for the strong topology, over the complex numbers, or as an étale morphism in general (under some slightly stronger hypotheses, on flatness and separability). Generically, then, such a morphism resembles a covering space in the topological sense. For example, if and are both Riemann surfaces, we require only that is holomorphic and not constant, and then there is a finite set of points of , outside of which we do find an honest covering
The set of exceptional points on is called the ramification locus (i.e. this is the complement of the largest possible open set ). In general monodromy occurs according to the fundamental group of acting on the sheets of the covering (this topological picture can be made precise also in the case of a general base field).
An unramified covering then is the occurrence of an empty ramification locus.
Morphisms of curves provide many examples of ramified coverings. For example, let C be the elliptic curve of equation
The projection of C onto the x-axis is a ramified cover with ramification locus given by
This is because for these three values of x the fiber is the double point while for any other value of x, the fiber consists of two distinct points (over an algebraically closed field).
Hence this projection is a degree 2 branched covering. This can be homogenized to construct a degree 2 branched covering of the corresponding projective elliptic curve to the projective line.
The previous example may be generalized to any algebraic plane curve in the following way. Let C be a plane curve defined by the equation f(x,y) = 0, where f is a separable and irreducible polynomial in two indeterminates. If n is the degree of f in y, then the fiber consists of n distinct points, except for a finite number of values of x. Thus, this projection is a branched covering of degree n.
The exceptional values of x are the roots of the coefficient of in f, and the roots of the discriminant of f with respect to y.
Over a root r of the discriminant, there is at least a ramified point, which is either a critical point or a singular point. If r is also a root of the coefficient of in f, then this ramified point is "at infinity".
Over a root s of the coefficient of in f, the curve C has an infinite branch, and the fiber at s has less than n points. However, if one extends the projection to the projective completions of C and the x-axis, and if s is not a root of the discriminant, the projection becomes a covering over a neighbourhood of s.
We can also generalize branched coverings of the line with varying ramification degrees. Consider a polynomial of the form
as we choose different points , the fibers given by the vanishing locus of vary. At any point where the multiplicity of one of the linear terms in the factorization of increases by one, there is a ramification.
Morphisms of curves provide many examples of ramified coverings of schemes. For example, the morphism from an affine elliptic curve to a line
is a ramified cover with ramification locus given by
This is because at any point of in the fiber is the scheme
Also, if we take the fraction fields of the underlying commutative rings, we get the field homomorphism
which is an algebraic extension of degree two; hence we got a degree 2 branched covering of an elliptic curve to the affine line. This can be homogenized to construct a morphism of a projective elliptic curve to .
A hyperelliptic curve provides a generalization of the above degree cover of the affine line, by considering the affine scheme defined over by a polynomial of the form
We can generalize the previous example by taking the morphism
where has no repeated roots. Then the ramification locus is given by
where the fibers are given by
Then, we get an induced morphism of fraction fields
There is an -module isomorphism of the target with
Hence the cover is of degree .
Superelliptic curves are a generalization of hyperelliptic curves and a specialization of the previous family of examples since they are given by affine schemes from polynomials of the form
Another useful class of examples come from ramified coverings of projective space. Given a homogeneous polynomial we can construct a ramified covering of with ramification locus
by considering the morphism of projective schemes
Again, this will be a covering of degree .
Branched coverings come with a symmetry group of transformations . Since the symmetry group has stabilizers at the points of the ramification locus, branched coverings can be used to construct examples of orbifolds, or Deligne-Mumford stacks.