In mathematics, an isomorphism is a structure-preserving mapping or morphism between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them, and this is often denoted as A ≅ B {\displaystyle A\cong B} . The word is derived from Ancient Greek ἴσος (isos) 'equal' and μορφή (morphe) 'form, shape'. The interest in isomorphisms lies in the fact that two isomorphic objects have the same properties (excluding further information such as additional structure or names of objects). Thus isomorphic structures cannot be distinguished from the point of view of structure only, and may often be identified, that is, considered the same. One says says also that the two structure are the same up to an isomorphism. For example, there are several different ways for defining the real numbers, and despite the different nature of the resulting objects (sequences of digits, subsets of the rational numbers, sequences of rational numbers, etc.), they are identified to form a unique set of the real numbers. On the other hand, a common case where isomorphic structures cannot be identified is when the structures are substructures of a larger one. For example, all subspaces of dimension one of a vector space are isomorphic and cannot be identified. An automorphism is an isomorphism from a structure to itself. An isomorphism between two structures is a canonical isomorphism (a canonical map that is an isomorphism) if there is only one isomorphism between the two structures (as is the case for solutions of a universal property), or if the isomorphism is much more natural (in some sense) than other isomorphisms. For example, for every prime number p, all fields with p elements are canonically isomorphic, with a unique isomorphism. The isomorphism theorems provide canonical isomorphisms that are not unique. The term isomorphism is mainly used for algebraic structures and categories. In the case of algebraic structures, mappings are called homomorphisms, and a homomorphism is an isomorphism if and only if it is bijective. In various areas of mathematics, isomorphisms have received specialized names, depending on the type of structure under consideration. For example:
An isometry is an isomorphism of metric spaces. A homeomorphism is an isomorphism of topological spaces. A diffeomorphism is an isomorphism of spaces equipped with a differential structure, typically differentiable manifolds. A symplectomorphism is an isomorphism of symplectic manifolds. A permutation is an automorphism of a set. In geometry, isomorphisms and automorphisms are often called transformations, for example rigid transformations, affine transformations, projective transformations. Category theory, which can be viewed as a formalization of the concept of mapping between structures, provides a language that may be used to unify the approach to these different aspects of the basic idea.
Examples
Logarithm and exponential Let R + {\displaystyle \mathbb {R} ^{+}} be the multiplicative group of positive real numbers, and let R {\displaystyle \mathbb {R} } be the additive group of real numbers. The logarithm function log : R + → R {\displaystyle \log :\mathbb {R} ^{+}\to \mathbb {R} } satisfies log ( x y ) = log x + log y {\displaystyle \log(xy)=\log x+\log y} for all x , y ∈ R + , {\displaystyle x,y\in \mathbb {R} ^{+},} so it is a group homomorphism. The exponential function exp : R → R + {\displaystyle \exp :\mathbb {R} \to \mathbb {R} ^{+}} satisfies exp ( x + y ) = ( exp x ) ( exp y ) {\displaystyle \exp(x+y)=(\exp x)(\exp y)} for all x , y ∈ R , {\displaystyle x,y\in \mathbb {R} ,} so it too is a homomorphism. The identities log exp x = x {\displaystyle \log \exp x=x} and exp log y = y {\displaystyle \exp \log y=y} show that log {\displaystyle \log } and exp {\displaystyle \exp } are inverses of each other. So,
exp : R → R + and log : R + → R {\displaystyle \exp :\mathbb {R} \to \mathbb {R} ^{+}\quad {\text{and}}\quad \log :\mathbb {R} ^{+}\to \mathbb {R} }
are group isomorphisms that are inverse of each other. The log {\displaystyle \log } function is an isomorphism which translates multiplication of positive real numbers into addition of real numbers. This facility makes it possible to multiply real numbers using a ruler and a table of logarithms, or using a slide rule with a logarithmic scale.
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