In
functional analysis
Functional analysis is a branch of mathematical analysis, the core of which is formed by the study of vector spaces endowed with some kind of limit-related structure (e.g. Inner product space#Definition, inner product, Norm (mathematics)#Defini ...
, a branch of mathematics, nest algebras are a class of
operator algebras that generalise the
upper-triangular matrix
In mathematics, a triangular matrix is a special kind of square matrix. A square matrix is called if all the entries ''above'' the main diagonal are zero. Similarly, a square matrix is called if all the entries ''below'' the main diagonal are ...
algebras to a
Hilbert space
In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...
context. They were introduced by and have many interesting properties. They are non-
selfadjoint algebras, are
closed
Closed may refer to:
Mathematics
* Closure (mathematics), a set, along with operations, for which applying those operations on members always results in a member of the set
* Closed set, a set which contains all its limit points
* Closed interval, ...
in the
weak operator topology
In functional analysis, the weak operator topology, often abbreviated WOT, is the weakest topology on the set of bounded operators on a Hilbert space H, such that the functional sending an operator T to the complex number \langle Tx, y\rangle is ...
and are
reflexive.
Nest algebras are among the simplest examples of
commutative subspace lattice algebra
In mathematics, a binary operation is commutative if changing the order of the operands does not change the result. It is a fundamental property of many binary operations, and many mathematical proofs depend on it. Most familiar as the name of ...
s. Indeed, they are formally defined as the algebra of
bounded operators leaving
invariant
Invariant and invariance may refer to:
Computer science
* Invariant (computer science), an expression whose value doesn't change during program execution
** Loop invariant, a property of a program loop that is true before (and after) each iteratio ...
each
subspace contained in a
subspace nest, that is, a set of subspaces which is
totally ordered
In mathematics, a total or linear order is a partial order in which any two elements are comparable. That is, a total order is a binary relation \leq on some set X, which satisfies the following for all a, b and c in X:
# a \leq a ( reflexive) ...
by
inclusion
Inclusion or Include may refer to:
Sociology
* Social inclusion, aims to create an environment that supports equal opportunity for individuals and groups that form a society.
** Inclusion (disability rights), promotion of people with disabilitie ...
and is also a
complete lattice
In mathematics, a complete lattice is a partially ordered set in which ''all'' subsets have both a supremum (join) and an infimum (meet). A lattice which satisfies at least one of these properties is known as a ''conditionally complete lattice.'' ...
. Since the
orthogonal projections corresponding to the subspaces in a nest
commute
Commute, commutation or commutative may refer to:
* Commuting, the process of travelling between a place of residence and a place of work
Mathematics
* Commutative property, a property of a mathematical operation whose result is insensitive to th ...
, nests are commutative subspace lattices.
By way of an example, let us apply this definition to recover the finite-dimensional upper-triangular matrices. Let us work in the
-
dimension
In physics and mathematics, the dimension of a Space (mathematics), mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any Point (geometry), point within it. Thus, a Line (geometry), lin ...
al
complex vector space
In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called ''vectors'', may be added together and multiplied ("scaled") by numbers called '' scalars''. Scalars are often real numbers, but can ...
, and let
be the
standard basis
In mathematics, the standard basis (also called natural basis or canonical basis) of a coordinate vector space (such as \mathbb^n or \mathbb^n) is the set of vectors whose components are all zero, except one that equals 1. For example, in the c ...
. For
, let
be the
-dimensional subspace of
span
Span may refer to:
Science, technology and engineering
* Span (unit), the width of a human hand
* Span (engineering), a section between two intermediate supports
* Wingspan, the distance between the wingtips of a bird or aircraft
* Sorbitan es ...
ned by the first
basis vectors
. Let
:
then ''N'' is a subspace nest, and the corresponding nest algebra of ''n'' × ''n'' complex matrices ''M'' leaving each subspace in ''N'' invariant that is, satisfying
for each ''S'' in ''N'' – is precisely the set of upper-triangular matrices.
If we omit one or more of the subspaces ''S
j'' from ''N'' then the corresponding nest algebra consists of block upper-triangular matrices.
Properties
* Nest algebras are
hyperreflexive In functional analysis, a reflexive operator algebra ''A'' is an operator algebra that has enough invariant subspaces to characterize it. Formally, ''A'' is reflexive if it is equal to the algebra of bounded operators which leave invariant each s ...
with distance constant 1.
See also
*
flag manifold
References
*
{{DEFAULTSORT:Nest Algebra
Operator theory
Operator algebras