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The notion of pullback in mathematics is a fundamental one. It refers to two different, but related processes: precomposition and fiber-product.
## Precomposition

Precomposition with a function probably provides the most elementary notion of pullback: in simple terms, a function f of a variable y, where y itself is a function of another variable x, may be written as a function of x. This is the pullback of f by the function y(x). It is such a fundamental process, that it is often passed over without mention, for instance in elementary calculus: this is sometimes called omitting pullbacks, and pervades areas as diverse as fluid mechanics and differential geometry.## Fibre-product

The notion of pullback as a fibre-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, and pullback bundles in algebraic topology and differential geometry.## Relationship

The relation between the two notions of pullback can perhaps best be illustrated by sections of fibre bundles: if s is a section of a fibre bundle E over N, and f is a map from M to N, then the pullback (precomposition) $f^*\; s=scirc\; f$ of s with f is a section of the pullback (fibre-product) bundle f*E over M.

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:

See:

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Last updated on Thursday July 03, 2008 at 11:33:00 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Thursday July 03, 2008 at 11:33:00 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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