Glossary of functional analysis

This is a glossary for the terminology in a mathematical field of functional analysis.

Throughout the article, unless stated otherwise, the base field of a vector space is the field of real numbers or that of complex numbers. Algebras are not assumed to be unital.

See also: List of Banach spaces, glossary of real and complex analysis.

*
*-homomorphism between involutive Banach algebras is an algebra homomorphism preserving *.
abelian
Synonymous with "commutative"; e.g., an abelian Banach algebra means a commutative Banach algebra.
Anderson–Kadec
The Anderson–Kadec theorem says a separable infinite-dimensional Fréchet space is isomorphic to  .
Alaoglu
Alaoglu's theorem states that the closed unit ball in a normed space is compact in the weak-* topology.
adjoint
The adjoint of a bounded linear operator   between Hilbert spaces is the bounded linear operator   such that   for each  .
approximate identity
In a not-necessarily-unital Banach algebra, an approximate identity is a sequence or a net   of elements such that   as   for each x in the algebra.
approximation property
A Banach space is said to have the approximation property if every compact operator is a limit of finite-rank operators.
Baire
The Baire category theorem states that a complete metric space is a Baire space; if   is a sequence of open dense subsets, then   is dense.
Banach
1.  A Banach space is a normed vector space that is complete as a metric space.
2.  A Banach algebra is a Banach space that has a structure of a possibly non-unital associative algebra such that
  for every   in the algebra.
3.  A Banach disc is a continuous linear image of a unit ball in a Banach space.
balanced
A subset S of a vector space over real or complex numbers is balanced if   for every scalar   of length at most one.
barrel
1.  A barrel in a topological vector space is a subset that is closed, convex, balanced and absorbing.
2.  A topological vector space is barrelled if every barrell is a neighborhood of zero (that is, contains an open neighborhood of zero).
Bessel
Bessel's inequality states: given an orthonormal set S and a vector x in a Hilbert space,
 ,[1]
where the equality holds if and only if S is an orthonormal basis; i.e., maximal orthonormal set.
bipolar
bipolar theorem.
bounded
A bounded operator is a linear operator between Banach spaces for which the image of the unit ball is bounded.
bornological
A bornological space.
Birkhoff orthogonality
Two vectors x and y in a normed linear space are said to be Birkhoff orthogonal if   for all scalars λ. If the normed linear space is a Hilbert space, then it is equivalent to the usual orthogonality.
Borel
Borel functional calculus
c
c space.
Calkin
The Calkin algebra on a Hilbert space is the quotient of the algebra of all bounded operators on the Hilbert space by the ideal generated by compact operators.
Cauchy–Schwarz inequality
The Cauchy–Schwarz inequality states: for each pair of vectors   in an inner-product space,
 .
closed
1.  The closed graph theorem states that a linear operator between Banach spaces is continuous (bounded) if and only if it has closed graph.
2.  A closed operator is a linear operator whose graph is closed.
3.  The closed range theorem says that a densely defined closed operator has closed image (range) if and only if the transpose of it has closed image.
commutant
1.  Another name for "centralizer"; i.e., the commutant of a subset S of an algebra is the algebra of the elements commuting with each element of S and is denoted by  .
2.  The von Neumann double commutant theorem states that a nondegenerate *-algebra   of operators on a Hilbert space is a von Neumann algebra if and only if  .
compact
A compact operator is a linear operator between Banach spaces for which the image of the unit ball is precompact.
Connes
Connes fusion.
C*
A C* algebra is an involutive Banach algebra satisfying  .
convex
A locally convex space is a topological vector space whose topology is generated by convex subsets.
cyclic
Given a representation   of a Banach algebra  , a cyclic vector is a vector   such that   is dense in  .
dilation
dilation (operator theory).
direct
Philosophically, a direct integral is a continuous analog of a direct sum.
Douglas
Douglas' lemma
Dunford
Dunford–Schwartz theorem
dual
1.  The continuous dual of a topological vector space is the vector space of all the continuous linear functionals on the space.
2.  The algebraic dual of a topological vector space is the dual vector space of the underlying vector space.
Eidelheit
A theorem of Eidelheit.
factor
A factor is a von Neumann algebra with trivial center.
faithful
A linear functional   on an involutive algebra is faithful if   for each nonzero element   in the algebra.
Fréchet
A Fréchet space is a topological vector space whose topology is given by a countable family of seminorms (which makes it a metric space) and that is complete as a metric space.
Fredholm
A Fredholm operator is a bounded operator such that it has closed range and the kernels of the operator and the adjoint have finite-dimension.
Gelfand
1.  The Gelfand–Mazur theorem states that a Banach algebra that is a division ring is the field of complex numbers.
2.  The Gelfand representation of a commutative Banach algebra   with spectrum   is the algebra homomorphism  , where   denotes the algebra of continuous functions on   vanishing at infinity, that is given by  . It is a *-preserving isometric isomorphism if   is a commutative C*-algebra.
Grothendieck
1.  Grothendieck's inequality.
2.  Grothendieck's factorization theorem.
Hahn–Banach
The Hahn–Banach theorem states: given a linear functional   on a subspace of a complex vector space V, if the absolute value of   is bounded above by a seminorm on V, then it extends to a linear functional on V still bounded by the seminorm. Geometrically, it is a generalization of the hyperplane separation theorem.
Heine
A topological vector space is said to have the Heine–Borel property if every closed and bounded subset is compact. Riesz's lemma says a Banach space with the Heine–Borel property must be finite-dimensional.
Hilbert
1.  A Hilbert space is an inner product space that is complete as a metric space.
2.  In the Tomita–Takesaki theory, a (left or right) Hilbert algebra is a certain algebra with an involution.
Hilbert–Schmidt
1.  The Hilbert–Schmidt norm of a bounded operator   on a Hilbert space is   where   is an orthonormal basis of the Hilbert space.
2.  A Hilbert–Schmidt operator is a bounded operator with finite Hilbert–Schmidt norm.
index
1.  The index of a Fredholm operator   is the integer  .
2.  The Atiyah–Singer index theorem.
index group
The index group of a unital Banach algebra is the quotient group   where   is the unit group of A and   the identity component of the group.
inner product
1.  An inner product on a real or complex vector space   is a function   such that for each  , (1)   is linear and (2)   where the bar means complex conjugate.
2.  An inner product space is a vector space equipped with an inner product.
involution
1.  An involution of a Banach algebra A is an isometric endomorphism   that is conjugate-linear and such that  .
2.  An involutive Banach algebra is a Banach algebra equipped with an involution.
isometry
A linear isometry between normed vector spaces is a linear map preserving norm.
Köthe
A Köthe sequence space. For now, see https://mathoverflow.net/questions/361048/on-k%C3%B6the-sequence-spaces
Krein–Milman
The Krein–Milman theorem states: a nonempty compact convex subset of a locally convex space has an extremal point.
Krein–Smulian
Krein–Smulian theorem
Linear
Linear Operators is a three-value book by Dunford and Schwartz.
Locally convex algebra
A locally convex algebra is an algebra whose underlying vector space is a locally convex space and whose multiplication is continuous with respect to the locally convex space topology.
Mazur
Mazur–Ulam theorem.
Montel
Montel space.
nondegenerate
A representation   of an algebra   is said to be nondegenerate if for each vector  , there is an element   such that  .
noncommutative
1.  noncommutative integration
2.  noncommutative torus
norm
1.  A norm on a vector space X is a real-valued function   such that for each scalar   and vectors   in  , (1)  , (2) (triangular inequality)   and (3)   where the equality holds only for  .
2.  A normed vector space is a real or complex vector space equipped with a norm  . It is a metric space with the distance function  .
normal
An operator is normal if it and its adjoint commute.
nuclear
See nuclear operator.
one
A one parameter group of a unital Banach algebra A is a continuous group homomorphism from   to the unit group of A.
open
The open mapping theorem says a surjective continuous linear operator between Banach spaces is an open mapping.
orthonormal
1.  A subset S of a Hilbert space is orthonormal if, for each u, v in the set,   = 0 when   and   when  .
2.  An orthonormal basis is a maximal orthonormal set (note: it is *not* necessarily a vector space basis.)
orthogonal
1.  Given a Hilbert space H and a closed subspace M, the orthogonal complement of M is the closed subspace  .
2.  In the notations above, the orthogonal projection   onto M is a (unique) bounded operator on H such that  
Parseval
Parseval's identity states: given an orthonormal basis S in a Hilbert space,  .[1]
positive
A linear functional   on an involutive Banach algebra is said to be positive if   for each element   in the algebra.
predual
predual.
projection
An operator T is called a projection if it is an idempotent; i.e.,  .
quasitrace
Quasitrace.
Radon
See Radon measure.
Riesz decomposition
Riesz decomposition.
Riesz's lemma
Riesz's lemma.
reflexive
A reflexive space is a topological vector space such that the natural map from the vector space to the second (topological) dual is an isomorphism.
resolvent
The resolvent of an element x of a unital Banach algebra is the complement in   of the spectrum of x.
Ryll-Nardzewski
Ryll-Nardzewski fixed-point theorem.
Schauder
Schauder basis.
Schatten
Schatten class
selection
Michael selection theorem.
self-adjoint
A self-adjoint operator is a bounded operator whose adjoint is itself.
separable
A separable Hilbert space is a Hilbert space admitting a finite or countable orthonormal basis.
spectrum
1.  The spectrum of an element x of a unital Banach algebra is the set of complex numbers   such that   is not invertible.
2.  The spectrum of a commutative Banach algebra is the set of all characters (a homomorphism to  ) on the algebra.
spectral
1.  The spectral radius of an element x of a unital Banach algebra is   where the sup is over the spectrum of x.
2.  The spectral mapping theorem states: if x is an element of a unital Banach algebra and f is a holomorphic function in a neighborhood of the spectrum   of x, then  , where   is an element of the Banach algebra defined via the Cauchy's integral formula.
state
A state is a positive linear functional of norm one.
symmetric
A linear operator T on a pre-Hilbert space is symmetric if  
tensor product
1.  See topological tensor product. Note it is still somewhat of an open problem to define or work out a correct tensor product of topological vector spaces, including Banach spaces.
2.  A projective tensor product.
topological
1.  A topological vector space is a vector space equipped with a topology such that (1) the topology is Hausdorff and (2) the addition   as well as scalar multiplication   are continuous.
2.  A linear map   is called a topological homomorphism if   is an open mapping.
3.  A sequence   is called topologically exact if it is an exact sequence on the underlying vector spaces and, moreover, each   is a topological homomorphism.
ultraweak
ultraweak topology.
unbounded operator
An unbounded operator is a partially defined linear operator, usually defined on a dense subspace.
uniform boundedness principle
The uniform boundedness principle states: given a set of operators between Banach spaces, if  , sup over the set, for each x in the Banach space, then  .
unitary
1.  A unitary operator between Hilbert spaces is an invertible bounded linear operator such that the inverse is the adjoint of the operator.
2.  Two representations   of an involutive Banach algebra A on Hilbert spaces   are said to be unitarily equivalent if there is a unitary operator   such that   for each x in A.
von Neumann
1.  A von Neumann algebra.
2.  von Neumann's theorem.
3.  Von Neumann's inequality.
W*
A W*-algebra is a C*-algebra that admits a faithful representation on a Hilbert space such that the image of the representation is a von Neumann algebra.

References

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  1. ^ a b Here, the part of the assertion is   is well-defined; i.e., when S is infinite, for countable totally ordered subsets  ,   is independent of   and   denotes the common value.
  • Bourbaki, Espaces vectoriels topologiques
  • Connes, Alain (1994), Non-commutative geometry, Boston, MA: Academic Press, ISBN 978-0-12-185860-5
  • Conway, John B. (1990). A Course in Functional Analysis. Graduate Texts in Mathematics. Vol. 96 (2nd ed.). New York: Springer-Verlag. ISBN 978-0-387-97245-9. OCLC 21195908.
  • Dunford, Nelson; Schwartz, Jacob T. (1988). Linear Operators. Pure and applied mathematics. Vol. 1. New York: Wiley-Interscience. ISBN 978-0-471-60848-6. OCLC 18412261.
  • Rudin, Walter (1991). Functional Analysis. International Series in Pure and Applied Mathematics. Vol. 8 (Second ed.). New York, NY: McGraw-Hill Science/Engineering/Math. ISBN 978-0-07-054236-5. OCLC 21163277.
  • M. Takesaki, Theory of Operator Algebras I, Springer, 2001, 2nd printing of the first edition 1979.
  • Yoshida, Kôsaku (1980), Functional Analysis (sixth ed.), Springer

Further reading

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