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The preimage of an output value is the set of input values that produce . More generally, evaluating f {\displaystyle f} at each element of a given subset A {\displaystyle A} of its domain X {\displaystyle X} produces a set, called the " image of A {\displaystyle A} under (or through) f {\displaystyle f} ".
This is in direct analogy to the definition that a continuous function between topological spaces preserves the topological structure: the preimage of any open set is open. In real analysis, measurable functions are used in the definition of the Lebesgue integral.
Let : be any function. If is any set then its preimage := under is necessarily an -saturated set.In particular, every fiber of a map is an -saturated set.. The empty set = and the domain = are always saturated.
An ordered pair (,), where is a set and is a σ-algebra over , is called a measurable space. A function between two measurable spaces is called a measurable function if the preimage of every measurable set is measurable.
An interpretation of a structure M in a structure N with parameters (or without parameters, respectively) is a pair (,) where n is a natural number and is a surjective map from a subset of N n onto M such that the -preimage (more precisely the -preimage) of every set X ⊆ M k definable in M by a first-order formula without parameters is definable (in N) by a first-order formula with ...
The preimage in G of the center of G/Z is called the second center and these groups begin the upper central series. Generalizing the earlier comments about the socle, a finite p-group with order p n contains normal subgroups of order p i with 0 ≤ i ≤ n, and any normal subgroup of order p i is contained in the ith center Z i.
If and are the domain and image of , respectively, then the fibers of are the sets in {():} = {{: =}:}which is a partition of the domain set .Note that must be restricted to the image set of , since otherwise () would be the empty set which is not allowed in a partition.
For example, the preimage of {,} under the square function is the set {,,,}. By definition of a function, the image of an element x of the domain is always a single element of the codomain. However, the preimage f − 1 ( y ) {\displaystyle f^{-1}(y)} of an element y of the codomain may be empty or contain any number of elements.