In
mathematics, a pullback is either of two different, but related processes: precomposition and fiber-product. Its dual is a
pushforward.
Precomposition
Precomposition with a
function
Function or functionality may refer to:
Computing
* Function key, a type of key on computer keyboards
* Function model, a structured representation of processes in a system
* Function object or functor or functionoid, a concept of object-oriente ...
probably provides the most elementary notion of pullback: in simple terms, a function
of a variable
where
itself is a function of another variable
may be written as a function of
This is the pullback of
by the function
It is such a fundamental process that it is often passed over without mention.
However, it is not just functions that can be "pulled back" in this sense. Pullbacks can be applied to many other objects such as
differential forms and their
cohomology classes; see
*
Pullback (differential geometry)
*
Pullback (cohomology)
Fiber-product
The pullback bundle is an example that bridges the notion of a pullback as precomposition, and the notion of a pullback as a
Cartesian square. In that example, the base space of a
fiber bundle is pulled back, in the sense of precomposition, above. The fibers then travel along with the points in the base space at which they are anchored: the resulting new pullback bundle looks locally like a Cartesian product of the new base space, and the (unchanged) fiber. The pullback bundle then has two projections: one to the base space, the other to the fiber; the product of the two becomes coherent when treated as a
fiber product.
Generalizations and category theory
The notion of pullback as a fiber-product ultimately leads to the very general idea of a
categorical pullback, but it has important special cases: inverse image (and pullback) sheaves in
algebraic geometry
Algebraic geometry is a branch of mathematics, classically studying zeros of multivariate polynomials. Modern algebraic geometry is based on the use of abstract algebraic techniques, mainly from commutative algebra, for solving geometrica ...
, and
pullback bundles in
algebraic topology
Algebraic topology is a branch of mathematics that uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify ...
and differential geometry.
See also:
*
Pullback (category theory)
*
Fibred category
*
Inverse image sheaf
Functional analysis
When the pullback is studied as an operator acting on
function spaces, it becomes a
linear operator
In mathematics, and more specifically in linear algebra, a linear map (also called a linear mapping, linear transformation, vector space homomorphism, or in some contexts linear function) is a mapping V \to W between two vector spaces that pr ...
, and is known as the
transpose or
composition operator
In mathematics, the composition operator C_\phi with symbol \phi is a linear operator defined by the rule
C_\phi (f) = f \circ \phi
where f \circ \phi denotes function composition.
The study of composition operators is covered bAMS category 47B3 ...
. Its adjoint is the push-forward, or, in the context of
functional analysis, the
transfer operator.
Relationship
The relation between the two notions of pullback can perhaps best be illustrated by
sections of fiber bundles: if
is a section of a fiber bundle
over
and
then the pullback (precomposition)
of ''s'' with
is a section of the pullback (fiber-product) bundle
over
See also
*
References
{{reflist
Mathematical analysis