*-algebra
Template:Short description Template:Algebraic structures In mathematics, and more specifically in abstract algebra, a *-algebra (or involutive algebra; read as "star-algebra") is a mathematical structure consisting of two involutive rings Template:Mvar and Template:Mvar, where Template:Mvar is commutative and Template:Mvar has the structure of an associative algebra over Template:Mvar. Involutive algebras generalize the idea of a number system equipped with conjugation, for example the complex numbers and complex conjugation, matrices over the complex numbers and conjugate transpose, and linear operators over a Hilbert space and Hermitian adjoints. However, it may happen that an algebra admits no involution.Template:Efn Template:Wiktionary
Definitions
*-ring
Template:Ring theory sidebar In mathematics, a *-ring is a ring with a map Template:Math that is an antiautomorphism and an involution.
More precisely, Template:Math is required to satisfy the following properties:<ref>Template:Cite web</ref>
for all Template:Math in Template:Mvar.
This is also called an involutive ring, involutory ring, and ring with involution. The third axiom is implied by the second and fourth axioms, making it redundant.
Elements such that Template:Math are called self-adjoint.<ref name=":0">Template:Cite web</ref>
Archetypical examples of a *-ring are fields of complex numbers and algebraic numbers with complex conjugation as the involution. One can define a sesquilinear form over any *-ring.
Template:AnchorAlso, one can define *-versions of algebraic objects, such as ideal and subring, with the requirement to be *-invariant: Template:Math and so on.
*-rings are unrelated to star semirings in the theory of computation.
*-algebra
A *-algebra Template:Mvar is a *-ring,Template:Efn with involution * that is an associative algebra over a commutative *-ring Template:Mvar with involution Template:Mvar, such that Template:Math.<ref>Template:Nlab</ref>
The base *-ring Template:Mvar is often the complex numbers (with Template:Mvar acting as complex conjugation).
It follows from the axioms that * on Template:Mvar is conjugate-linear in Template:Mvar, meaning
for Template:Math.
A *-homomorphism Template:Math is an algebra homomorphism that is compatible with the involutions of Template:Mvar and Template:Mvar, i.e.,
- Template:Math for all Template:Mvar in Template:Mvar.<ref name=":0" />
Philosophy of the *-operation
The *-operation on a *-ring is analogous to complex conjugation on the complex numbers. The *-operation on a *-algebra is analogous to taking adjoints in complex matrix algebras.
Notation
The * involution is a unary operation written with a postfixed star glyph centered above or near the mean line:
- Template:Math, or
- Template:Math (TeX:
x^*),
but not as "Template:Math"; see the asterisk article for details.
Examples
- Any commutative ring becomes a *-ring with the trivial (identical) involution.
- The most familiar example of a *-ring and a *-algebra over reals is the field of complex numbers Template:Math where * is just complex conjugation.
- More generally, a field extension made by adjunction of a square root (such as the imaginary unit Template:Sqrt) is a *-algebra over the original field, considered as a trivially-*-ring. The * flips the sign of that square root.
- A quadratic integer ring (for some Template:Mvar) is a commutative *-ring with the * defined in the similar way; quadratic fields are *-algebras over appropriate quadratic integer rings.
- Quaternions, split-complex numbers, dual numbers, and possibly other hypercomplex number systems form *-rings (with their built-in conjugation operation) and *-algebras over reals (where * is trivial). None of the three is a complex algebra.
- Hurwitz quaternions form a non-commutative *-ring with the quaternion conjugation.
- The matrix algebra of Template:Mathmatrices over R with * given by the transposition.
- The matrix algebra of Template:Mathmatrices over C with * given by the conjugate transpose.
- Its generalization, the Hermitian adjoint in the algebra of bounded linear operators on a Hilbert space also defines a *-algebra.
- The polynomial ring Template:Math over a commutative trivially-*-ring Template:Mvar is a *-algebra over Template:Mvar with Template:Math.
- If Template:Math is simultaneously a *-ring, an algebra over a ring Template:Mvar (commutative), and Template:Math, then Template:Mvar is a *-algebra over Template:Mvar (where * is trivial).
- As a partial case, any *-ring is a *-algebra over integers.
- Any commutative *-ring is a *-algebra over itself and, more generally, over any its *-subring.
- For a commutative *-ring Template:Mvar, its quotient by any its *-ideal is a *-algebra over Template:Mvar.
- For example, any commutative trivially-*-ring is a *-algebra over its dual numbers ring, a *-ring with non-trivial *, because the quotient by Template:Math makes the original ring.
- The same about a commutative ring Template:Mvar and its polynomial ring Template:Math: the quotient by Template:Math restores Template:Mvar.
- In Hecke algebra, an involution is important to the Kazhdan–Lusztig polynomial.
- The endomorphism ring of an elliptic curve becomes a *-algebra over the integers, where the involution is given by taking the dual isogeny. A similar construction works for abelian varieties with a polarization, in which case it is called the Rosati involution (see Milne's lecture notes on abelian varieties).
Involutive Hopf algebras are important examples of *-algebras (with the additional structure of a compatible comultiplication); the most familiar example being:
- The group Hopf algebra: a group ring, with involution given by Template:Math.
Non-Example
Not every algebra admits an involution:
Regard the 2×2 matrices over the complex numbers. Consider the following subalgebra: <math display="block">\mathcal{A} := \left\{\begin{pmatrix}a&b\\0&0\end{pmatrix} : a,b\in\Complex\right\}</math>
Any nontrivial antiautomorphism necessarily has the form:<ref>Template:Cite journal</ref> <math display="block">\varphi_z\left[\begin{pmatrix}1&0\\0&0\end{pmatrix}\right] = \begin{pmatrix}1&z\\0&0\end{pmatrix} \quad \varphi_z\left[\begin{pmatrix}0&1\\0&0\end{pmatrix}\right] = \begin{pmatrix}0&0\\0&0\end{pmatrix}</math> for any complex number <math>z\in\Complex</math>.
It follows that any nontrivial antiautomorphism fails to be involutive: <math display="block">\varphi_z^2\left[\begin{pmatrix}0&1\\0&0\end{pmatrix}\right] = \begin{pmatrix}0&0\\0&0\end{pmatrix}\neq\begin{pmatrix}0&1\\0&0\end{pmatrix}</math>
Concluding that the subalgebra admits no involution.
Additional structures
Many properties of the transpose hold for general *-algebras:
- The Hermitian elements form a Jordan algebra;
- The skew Hermitian elements form a Lie algebra;
- If 2 is invertible in the *-ring, then the operators Template:Math and Template:Math are orthogonal idempotents,<ref name=":0" /> called symmetrizing and anti-symmetrizing, so the algebra decomposes as a direct sum of modules (vector spaces if the *-ring is a field) of symmetric and anti-symmetric (Hermitian and skew Hermitian) elements. These spaces do not, generally, form associative algebras, because the idempotents are operators, not elements of the algebra.
Skew structures
Given a *-ring, there is also the map Template:Math. It does not define a *-ring structure (unless the characteristic is 2, in which case −* is identical to the original *), as Template:Math, neither is it antimultiplicative, but it satisfies the other axioms (linear, involution) and hence is quite similar to *-algebra where Template:Math.
Elements fixed by this map (i.e., such that Template:Math) are called skew Hermitian.
For the complex numbers with complex conjugation, the real numbers are the Hermitian elements, and the imaginary numbers are the skew Hermitian.
See also
- Semigroup with involution
- B*-algebra
- C*-algebra
- Dagger category
- von Neumann algebra
- Baer ring
- Operator algebra
- Conjugate (algebra)
- Cayley–Dickson construction
- Composition algebra