In mathematics, an **isomorphism** is a mapping 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. The word isomorphism is derived from the Ancient Greek: ἴσος *isos* "equal", and μορφή *morphe* "form" or "shape".

- Examples
- Logarithm and exponential
- Integers modulo 6
- Relation-preserving isomorphism
- Applications
- Category theoretic view
- Isomorphism vs. bijective morphism
- Relation with equality
- See also
- Notes
- References
- Further reading
- External links

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 be identified. In mathematical jargon, one says that two objects are *the same up to an isomorphism*.

An automorphism is an isomorphism from a structure to itself. An isomorphism between two structures is a **canonical isomorphism** if there is only one isomorphism between the two structures (as it 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. In this case, 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 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.

Let be the multiplicative group of positive real numbers, and let be the additive group of real numbers.

The logarithm function satisfies for all , so it is a group homomorphism. The exponential function satisfies for all , so it too is a homomorphism.

The identities and show that and are inverses of each other. Since is a homomorphism that has an inverse that is also a homomorphism, is an isomorphism of groups.

The 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.

Consider the group , the integers from 0 to 5 with addition modulo 6. Also consider the group , the ordered pairs where the *x* coordinates can be 0 or 1, and the y coordinates can be 0, 1, or 2, where addition in the *x*-coordinate is modulo 2 and addition in the *y*-coordinate is modulo 3.

These structures are isomorphic under addition, under the following scheme:

- (0,0) ↦ 0
- (1,1) ↦ 1
- (0,2) ↦ 2
- (1,0) ↦ 3
- (0,1) ↦ 4
- (1,2) ↦ 5

or in general (*a*,*b*) ↦ (3*a* + 4*b*) mod 6.

For example, (1,1) + (1,0) = (0,1), which translates in the other system as 1 + 3 = 4.

Even though these two groups "look" different in that the sets contain different elements, they are indeed **isomorphic**: their structures are exactly the same. More generally, the direct product of two cyclic groups and is isomorphic to if and only if *m* and *n* are coprime, per the Chinese remainder theorem.

If one object consists of a set *X* with a binary relation R and the other object consists of a set *Y* with a binary relation S then an isomorphism from *X* to *Y* is a bijective function ƒ: *X* → *Y* such that:^{ [1] }

S is reflexive, irreflexive, symmetric, antisymmetric, asymmetric, transitive, total, trichotomous, a partial order, total order, well-order, strict weak order, total preorder (weak order), an equivalence relation, or a relation with any other special properties, if and only if R is.

For example, R is an ordering ≤ and S an ordering , then an isomorphism from *X* to *Y* is a bijective function ƒ: *X* → *Y* such that

Such an isomorphism is called an * order isomorphism * or (less commonly) an *isotone isomorphism*.

If *X* = *Y*, then this is a relation-preserving automorphism.

In abstract algebra, two basic isomorphisms are defined:

- Group isomorphism, an isomorphism between groups
- Ring isomorphism, an isomorphism between rings. (Isomorphisms between fields are actually ring isomorphisms)

Just as the automorphisms of an algebraic structure form a group, the isomorphisms between two algebras sharing a common structure form a heap. Letting a particular isomorphism identify the two structures turns this heap into a group.

In mathematical analysis, the Laplace transform is an isomorphism mapping hard differential equations into easier algebraic equations.

In graph theory, an isomorphism between two graphs *G* and *H* is a bijective map *f* from the vertices of *G* to the vertices of *H* that preserves the "edge structure" in the sense that there is an edge from vertex *u* to vertex *v* in *G* if and only if there is an edge from ƒ(*u*) to ƒ(*v*) in *H*. See graph isomorphism.

In mathematical analysis, an isomorphism between two Hilbert spaces is a bijection preserving addition, scalar multiplication, and inner product.

In early theories of logical atomism, the formal relationship between facts and true propositions was theorized by Bertrand Russell and Ludwig Wittgenstein to be isomorphic. An example of this line of thinking can be found in Russell's * Introduction to Mathematical Philosophy *.

In cybernetics, the good regulator or Conant–Ashby theorem is stated "Every good regulator of a system must be a model of that system". Whether regulated or self-regulating, an isomorphism is required between the regulator and processing parts of the system.

In category theory, given a category *C*, an isomorphism is a morphism *f*: *a* → *b* that has an inverse morphism *g*: *b* → *a*, that is, *fg* = 1_{b} and *gf* = 1_{a}. For example, a bijective linear map is an isomorphism between vector spaces, and a bijective continuous function whose inverse is also continuous is an isomorphism between topological spaces, called a homeomorphism.

In a concrete category (that is, a category whose objects are sets (perhaps with extra structure) and whose morphisms are structure-preserving functions), such as the category of topological spaces or categories of algebraic objects like groups, rings, and modules, an isomorphism must be bijective on the underlying sets. In algebraic categories (specifically, categories of varieties in the sense of universal algebra), an isomorphism is the same as a homomorphism which is bijective on underlying sets. However, there are concrete categories in which bijective morphisms are not necessarily isomorphisms (such as the category of topological spaces).

In certain areas of mathematics, notably category theory, it is valuable to distinguish between * equality * on the one hand and *isomorphism* on the other.^{ [2] } Equality is when two objects are exactly the same, and everything that's true about one object is true about the other, while an isomorphism implies everything that's true about a designated part of one object's structure is true about the other's. For example, the sets

- and

are *equal*; they are merely different representations—the first an intensional one (in set builder notation), and the second extensional (by explicit enumeration)—of the same subset of the integers. By contrast, the sets {*A*,*B*,*C*} and {1,2,3} are not *equal*—the first has elements that are letters, while the second has elements that are numbers. These are isomorphic as sets, since finite sets are determined up to isomorphism by their cardinality (number of elements) and these both have three elements, but there are many choices of isomorphism—one isomorphism is

- while another is

and no one isomorphism is intrinsically better than any other.^{ [note 1] }^{ [note 2] } On this view and in this sense, these two sets are not equal because one cannot consider them *identical*: one can choose an isomorphism between them, but that is a weaker claim than identity—and valid only in the context of the chosen isomorphism.

Sometimes the isomorphisms can seem obvious and compelling, but are still not equalities. As a simple example, the genealogical relationships among Joe, John, and Bobby Kennedy are, in a real sense, the same as those among the American football quarterbacks in the Manning family: Archie, Peyton, and Eli. The father-son pairings and the elder-brother-younger-brother pairings correspond perfectly. That similarity between the two family structures illustrates the origin of the word *isomorphism* (Greek *iso*-, "same," and -*morph*, "form" or "shape"). But because the Kennedys are not the same people as the Mannings, the two genealogical structures are merely isomorphic and not equal.

Another example is more formal and more directly illustrates the motivation for distinguishing equality from isomorphism: the distinction between a finite-dimensional vector space *V* and its dual space *V** = { φ: V → **K**} of linear maps from *V* to its field of scalars **K**. These spaces have the same dimension, and thus are isomorphic as abstract vector spaces (since algebraically, vector spaces are classified by dimension, just as sets are classified by cardinality), but there is no "natural" choice of isomorphism . If one chooses a basis for *V*, then this yields an isomorphism: For all *u*. *v* ∈ *V*,

- .

This corresponds to transforming a column vector (element of *V*) to a row vector (element of *V**) by transpose, but a different choice of basis gives a different isomorphism: the isomorphism "depends on the choice of basis". More subtly, there *is* a map from a vector space *V* to its double dual *V*** = { *x*: *V** → **K**} that does not depend on the choice of basis: For all *v* ∈ *V* and φ ∈ *V**,

- .

This leads to a third notion, that of a natural isomorphism: while *V* and *V*** are different sets, there is a "natural" choice of isomorphism between them. This intuitive notion of "an isomorphism that does not depend on an arbitrary choice" is formalized in the notion of a natural transformation; briefly, that one may *consistently* identify, or more generally map from, a finite-dimensional vector space to its double dual, , for *any* vector space in a consistent way. Formalizing this intuition is a motivation for the development of category theory.

However, there is a case where the distinction between natural isomorphism and equality is usually not made. That is for the objects that may be characterized by a universal property. In fact, there is a unique isomorphism, necessarily natural, between two objects sharing the same universal property. A typical example is the set of real numbers, which may be defined through infinite decimal expansion, infinite binary expansion, Cauchy sequences, Dedekind cuts and many other ways. Formally these constructions define different objects, which all are solutions of the same universal property. As these objects have exactly the same properties, one may forget the method of construction and considering them as equal. This is what everybody does when talking of "*the* set of the real numbers". The same occurs with quotient spaces: they are commonly constructed as sets of equivalence classes. However, talking of set of sets may be counterintuitive, and quotient spaces are commonly considered as a pair of a set of undetermined objects, often called "points", and a surjective map onto this set.

If one wishes to draw a distinction between an arbitrary isomorphism (one that depends on a choice) and a natural isomorphism (one that can be done consistently), one may write ≈ for an unnatural isomorphism and ≅ for a natural isomorphism, as in *V* ≈ *V** and *V*≅*V***. This convention is not universally followed, and authors who wish to distinguish between unnatural isomorphisms and natural isomorphisms will generally explicitly state the distinction.

Generally, saying that two objects are *equal* is reserved for when there is a notion of a larger (ambient) space that these objects live in. Most often, one speaks of equality of two subsets of a given set (as in the integer set example above), but not of two objects abstractly presented. For example, the 2-dimensional unit sphere in 3-dimensional space

- and the Riemann sphere

which can be presented as the one-point compactification of the complex plane **C** ∪ {∞} *or* as the complex projective line (a quotient space)

are three different descriptions for a mathematical object, all of which are isomorphic, but not *equal* because they are not all subsets of a single space: the first is a subset of **R**^{3}, the second is **C** ≅ **R**^{2}^{ [note 3] } plus an additional point, and the third is a subquotient of **C**^{2}

In the context of category theory, objects are usually at most isomorphic—indeed, a motivation for the development of category theory was showing that different constructions in homology theory yielded equivalent (isomorphic) groups. Given maps between two objects *X* and *Y*, however, one asks if they are equal or not (they are both elements of the set Hom(*X*, *Y*), hence equality is the proper relationship), particularly in commutative diagrams.

- ↑
*A*,*B*,*C*have a conventional order, namely alphabetical order, and similarly 1, 2, 3 have the order from the integers, and thus one particular isomorphism is "natural", namely- .

*sets*these are isomorphic, but not naturally isomorphic (there are multiple choices of isomorphism), while as*ordered sets*they are naturally isomorphic (there is a unique isomorphism, given above), since finite total orders are uniquely determined up to unique isomorphism by cardinality. This intuition can be formalized by saying that any two finite totally ordered sets of the same cardinality have a natural isomorphism, the one that sends the least element of the first to the least element of the second, the least element of what remains in the first to the least element of what remains in the second, and so forth, but in general, pairs of sets of a given finite cardinality are not naturally isomorphic because there is more than one choice of map—except if the cardinality is 0 or 1, where there is a unique choice. - ↑ In fact, there are precisely different isomorphisms between two sets with three elements. This is equal to the number of automorphisms of a given three-element set (which in turn is equal to the order of the symmetric group on three letters), and more generally one has that the set of isomorphisms between two objects, denoted is a torsor for the automorphism group of
*A,*and also a torsor for the automorphism group of*B.*In fact, automorphisms of an object are a key reason to be concerned with the distinction between isomorphism and equality, as demonstrated in the effect of change of basis on the identification of a vector space with its dual or with its double dual, as elaborated in the sequel. - ↑ Being precise, the identification of the complex numbers with the real plane,

In mathematics, an **automorphism** is an isomorphism from a mathematical object to itself. It is, in some sense, a symmetry of the object, and a way of mapping the object to itself while preserving all of its structure. The set of all automorphisms of an object forms a group, called the automorphism group. It is, loosely speaking, the symmetry group of the object.

A **quotient group** or **factor group** is a mathematical group obtained by aggregating similar elements of a larger group using an equivalence relation that preserves some of the group structure. For example, the cyclic group of addition modulo *n* can be obtained from the group of integers under addition by identifying elements that differ by a multiple of *n* and defining a group structure that operates on each such class as a single entity. It is part of the mathematical field known as group theory.

In mathematics, given two groups, and, a **group homomorphism** from to is a function *h* : *G* → *H* such that for all *u* and *v* in *G* it holds that

In abstract algebra, a **group isomorphism** is a function between two groups that sets up a one-to-one correspondence between the elements of the groups in a way that respects the given group operations. If there exists an isomorphism between two groups, then the groups are called **isomorphic**. From the standpoint of group theory, isomorphic groups have the same properties and need not be distinguished.

In mathematics, a **group action** on a space is a group homomorphism of a given group into the group of transformations of the space. Similarly, a group action on a mathematical structure is a group homomorphism of a group into the automorphism group of the structure. It is said that the group *acts* on the space or structure. If a group acts on a structure, it also acts on everything that is built on the structure. For example, the group of Euclidean isometries acts on Euclidean space and also on the figures drawn in it. In particular, it acts on the set of all triangles. Similarly, the group of symmetries of a polyhedron acts on the vertices, the edges, and the faces of the polyhedron.

In algebra, a **homomorphism** is a structure-preserving map between two algebraic structures of the same type. The word *homomorphism* comes from the ancient Greek language: *ὁμός (homos)* meaning "same" and *μορφή (morphe)* meaning "form" or "shape". However, the word was apparently introduced to mathematics due to a (mis)translation of German *ähnlich* meaning "similar" to *ὁμός* meaning "same". The term "homomorphism" appeared as early as 1892, when it was attributed to the German mathematician Felix Klein (1849–1925).

In mathematics, a **Lie group** is a group whose elements are organized continuously and smoothly, as opposed to discrete groups, where the elements are separated—this makes Lie groups differentiable manifolds. Classically, such groups were found by studying matrix subgroups contained in or , the group of invertible matrices over or . Lie groups are named after Norwegian mathematician Sophus Lie (1842–1899), who laid the foundations of the theory of continuous transformation groups.

In ring theory, a branch of abstract algebra, a **ring homomorphism** is a structure-preserving function between two rings. More explicitly, if *R* and *S* are rings, then a ring homomorphism is a function *f* : *R* → *S* such that *f* is

In category theory, a branch of mathematics, a **natural transformation** provides a way of transforming one functor into another while respecting the internal structure of the categories involved. Hence, a natural transformation can be considered to be a "morphism of functors". Indeed, this intuition can be formalized to define so-called functor categories. Natural transformations are, after categories and functors, one of the most fundamental notions of category theory and consequently appear in the majority of its applications.

In mathematics, a **ring** is one of the fundamental algebraic structures used in abstract algebra. It consists of a set equipped with two binary operations that generalize the arithmetic operations of addition and multiplication. Through this generalization, theorems from arithmetic are extended to non-numerical objects such as polynomials, series, matrices and functions.

In category theory, an **epimorphism** is a morphism *f* : *X* → *Y* that is right-cancellative in the sense that, for all objects *Z* and all morphisms *g*_{1}, *g*_{2}: *Y* → *Z*,

In mathematics, an **algebra over a field** is a vector space equipped with a bilinear product. Thus, an algebra is an algebraic structure, which consists of a set, together with operations of multiplication, addition, and scalar multiplication by elements of the underlying field, and satisfies the axioms implied by "vector space" and "bilinear".

In mathematics, an **algebra homomorphism** is an homomorphism between two associative algebras. More precisely, if *A* and *B*, are algebras over a field *K*, it is a function such that for all *k* in *K* and *x*, *y* in *A*,

In mathematics, specifically in harmonic analysis and the theory of topological groups, **Pontryagin duality** explains the general properties of the Fourier transform on locally compact abelian groups, such as , the circle, or finite cyclic groups. The **Pontryagin duality theorem** itself states that locally compact abelian groups identify naturally with their bidual.

In mathematics, especially in algebraic geometry and the theory of complex manifolds, **coherent sheaves** are a class of sheaves closely linked to the geometric properties of the underlying space. The definition of coherent sheaves is made with reference to a sheaf of rings that codifies this geometric information.

In mathematics, the **Grothendieck group** construction constructs an abelian group from a commutative monoid *M* in the most universal way, in the sense that any abelian group containing a homomorphic image of *M* will also contain a homomorphic image of the Grothendieck group of *M*. The Grothendieck group construction takes its name from a specific case in category theory, introduced by Alexander Grothendieck in his proof of the Grothendieck–Riemann–Roch theorem, which resulted in the development of K-theory. This specific case is the monoid of isomorphism classes of objects of an abelian category, with the direct sum as its operation.

In mathematics, a **split-biquaternion** is a hypercomplex number of the form

In mathematics, **Lie group–Lie algebra correspondence** allows one to study Lie groups, which are geometric objects, in terms of Lie algebras, which are linear objects. In this article, a Lie group refers to a real Lie group. For the complex and *p*-adic cases, see complex Lie group and *p*-adic Lie group.

In the theory of Lie groups, the **exponential map** is a map from the Lie algebra of a Lie group to the group, which allows one to recapture the local group structure from the Lie algebra. The existence of the exponential map is one of the primary reasons that Lie algebras are a useful tool for studying Lie groups.

In mathematics, the **automorphism group** of an object *X* is the group consisting of automorphisms of *X*. For example, if *X* is a finite-dimensional vector space, then the automorphism group of *X* is the general linear group of *X*, the group of invertible linear transformations from *X* to itself.

This article needs additional citations for verification .(September 2010) (Learn how and when to remove this template message) |

- ↑ Vinberg, Ėrnest Borisovich (2003).
*A Course in Algebra*. American Mathematical Society. p. 3. ISBN 9780821834138. - ↑ Mazur 2007

- Mazur, Barry (12 June 2007),
*When is one thing equal to some other thing?*(PDF)CS1 maint: ref=harv (link)

Look up in Wiktionary, the free dictionary. isomorphism |

- Hazewinkel, Michiel, ed. (2001) [1994], "Isomorphism",
*Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4 - "Isomorphism".
*PlanetMath*. - Weisstein, Eric W. "Isomorphism".
*MathWorld*.

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