Fiber product
From AoPSWiki
The fiber product, also called the pullback, is an idea in category theory which occurs in many areas of mathematics.
Definition
Let 
, 
, and 
be objects of the same category; let 
and 
be homomorphisms of this category. Then the fiber product of and with respect to , denoted 
(when the specific functions 
and 
are clear) is the set of elements 
in the product 
in which 
.
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