# involutory ring

## General Definition of a Ring with Involution

Let $R$ be a ring. An $*$ on $R$ is an anti-endomorphism whose square is the identity map. In other words, for $a,b\in R$:

1. 1.

$(a+b)^{*}=a^{*}+b^{*}$,

2. 2.

$(ab)^{*}=b^{*}a^{*}$,

3. 3.

$a^{**}=a$

A ring admitting an involution is called an involutory ring. $a^{*}$ is called the adjoint of $a$. By (3), $a$ is the adjoint of $a^{*}$, so that every element of $R$ is an adjoint.

Remark. Note that the traditional definition of an involution (http://planetmath.org/Involution) on a vector space is different from the one given here. Clearly, $*$ is bijective, so that it is an anti-automorphism. If $*$ is the identity on $R$, then $R$ is commutative.

Examples. Involutory rings occur most often in rings of endomorphisms over a module. When $V$ is a finite dimensional vector space over a field $k$ with a given basis $\boldsymbol{b}$, any linear transformation over $T$ (to itself) can be represented by a square matrix $M$ over $k$ via $\boldsymbol{b}$. The map taking $M$ to its transpose $M^{T}$ is an involution. If $k$ is $\mathbb{C}$, then the map taking $M$ to its conjugate transpose $\overline{M}^{T}$ is also an involution. In general, the composition of an isomorphism and an involution is an involution, and the composition of two involutions is an isomorphism.

## *-Homomorphisms

Let $R$ and $S$ be involutory rings with involutions $*_{R}$ and $*_{S}$. A *-homomorphism $\phi:R\to S$ is a ring homomorphism which respects involutions. More precisely,

 $\phi(a^{*_{R}})=\phi(a)^{*_{S}},\quad\mbox{ for any }a\in R.$

By abuse of notation, if we use $*$ to denote both $*_{R}$ and $*_{S}$, then we see that any *-homomorphism $\phi$ commutes with $*$: $\phi*=*\phi$.

## Special Elements

An element $a\in R$ such that $a=a^{*}$ is called a self-adjoint. A ring with involution is usually associated with a ring of square matrices over a field, as such, a self-adjoint element is sometimes called a Hermitian element, or a symmetric element. For example, for any element $a\in R$,

1. 1.

$aa^{*}$ and $a^{*}a$ are both self-adjoint, the first of which is called the norm of $a$. A norm element $b$ is simply an element expressible in the form $aa^{*}$ for some $a\in R$, and we write $b=\operatorname{n}(a)$. If $aa^{*}=a^{*}a$, then $a$ is called a normal element. If $a^{*}$ is the multiplicative inverse of $a$, then $a$ is a unitary element. If $a$ is unitary, then it is normal.

2. 2.

With respect to addition, we can also form self-adjoint elements $a+a^{*}=a^{*}+a$, called the trace of $a$, for any $a\in R$. A trace element $b$ is an element expressible as $a+a^{*}$ for some $a\in R$, and written $b=\operatorname{tr}(a)$.

Let $S$ be a subset of $R$, write $S^{*}:=\{a^{*}\mid a\in S\}$. Then $S$ is said to be self-adjoint if $S=S^{*}$.

A self-adjoint that is also an idempotent in $R$ is called a projection. If $e$ and $f$ are two projections in $R$ such that $eR=fR$ (principal ideals generated by $e$ and $f$ are equal), then $e=f$. For if $ea=ff=f$ for some $a\in R$, then $f=ea=eea=ef$. Similarly, $e=fe$. Therefore, $e=e^{*}=(fe)^{*}=e^{*}f^{*}=ef=f$.

If the characteristic of $R$ is not 2, we also have a companion concept to self-adjointness, that of skew symmetry. An element $a$ in $R$ is skew symmetric if $a=-a^{*}$. Again, the name of this is borrowed from linear algebra.

 Title involutory ring Canonical name InvolutoryRing Date of creation 2013-03-22 15:41:01 Last modified on 2013-03-22 15:41:01 Owner CWoo (3771) Last modified by CWoo (3771) Numerical id 32 Author CWoo (3771) Entry type Definition Classification msc 16W10 Synonym ring admitting an involution Synonym involutary ring Synonym involutive ring Synonym ring with involution Synonym Hermitian element Synonym symmetric element Synonym self-adjoint Synonym adjoint Synonym projection Synonym involutive ring Related topic HollowMatrixRings Defines involution Defines adjoint element Defines self-adjoint element Defines projection element Defines norm element Defines trace element Defines skew symmetric element Defines *-homomorphism Defines normal element Defines unitary element