Hilbert Spaces and Operator Theory

Oct 13, 2025 by wbrum.org 148 views

Introduction

In the realm of functional analysis, operator theory, and complex analysis, the concept of analytic continuation plays a vital role in understanding the behavior of functions defined on a complex domain. When dealing with functions of operators, it is essential to determine under what conditions these functions can be analytically continued. This article aims to explore the conditions under which a function of an operator can be analytically continued, providing a comprehensive overview of the underlying mathematical concepts.

Background

To begin with, let us recall the definition of analytic continuation. Given a function f(z) defined on a domain D, an analytic continuation of f(z) to a larger domain D' is a function F(z) defined on D' such that F(z) = f(z) for all z in D. In other words, an analytic continuation of a function is an extension of the function to a larger domain, where the extended function agrees with the original function on the original domain.

In the context of operator theory, an operator T is a linear transformation from a vector space X to itself. A function of an operator T is a function f(T) that maps the operator T to a scalar or another operator. The concept of analytic continuation for functions of operators is crucial in understanding the behavior of these functions, particularly in the study of spectral properties of operators.

Hilbert Spaces and Operator Theory

To discuss the conditions under which a function of an operator can be analytically continued, we need to introduce the concept of Hilbert spaces and operator theory. A Hilbert space is a complete inner product space, and it is a fundamental tool in functional analysis. In the context of operator theory, a Hilbert space is a vector space equipped with an inner product that satisfies certain properties.

Let H be a Hilbert space, and let T be a bounded linear operator on H. The operator T is said to be a Hilbert-Schmidt operator if the sum of the squares of the singular values of T is finite. The singular values of T are the square roots of the eigenvalues of the operator TT, where T is the adjoint of T.

Spectral Properties of Operators

The spectral properties of operators play a crucial role in understanding the behavior of functions of operators. The spectrum of an operator T is the set of all complex numbers λ such that the operator T - λI is not invertible, where I is the identity operator. The spectrum of an operator can be decomposed into three parts: the point spectrum, the continuous spectrum, and the residual spectrum.

The point spectrum of an operator T consists of all eigenvalues of T, while the continuous spectrum consists of all complex numbers λ such that the operator T - λI is not invertible but has a dense range. The residual spectrum consists of all complex numbers λ such that the operator T - λI is not invertible but has a non-trivial kernel.

Analytic Continuation of Functions of Operators

To determine under what conditions a function of an operator can be analytically continued, we need to consider the spectral properties of the operator. Let T be a bounded linear operator on a Hilbert space H, and let f(z) be a function defined on a domain D. We say that f(z) is analytic at a point z0 in D if the function f(z) can be expanded in a power series at z0.

The function f(z) is said to be analytic on a domain D if it is analytic at every point in D. In the context of operator theory, a function f(T) of an operator T is said to be analytic at a point z0 in the spectrum of T if the function f(z) can be expanded in a power series at z0.

Conditions for Analytic Continuation

To determine under what conditions a function of an operator can be analytically continued, we need to consider the following conditions:

Spectral properties of the operator: The function f(z) must be analytic at every point in the spectrum of the operator T.
Hilbert-Schmidt property: The operator T must be a Hilbert-Schmidt operator.
Boundedness of the operator: The operator T must be bounded.

If these conditions are satisfied, then the function f(z) can be analytically continued to a larger domain D', where the extended function agrees with the original function on the original domain D.

Examples and Applications

To illustrate the concept of analytic continuation for functions of operators, let us consider the following examples:

Resolvent operator: Let T be a bounded linear operator on a Hilbert space H, and let λ be a complex number. The resolvent operator R(λ) is defined as the inverse of the operator T - λI. The resolvent operator is analytic at every point in the spectrum of T, and it can be analytically continued to a larger domain D'.
Spectral function: Let T be a bounded linear operator on a Hilbert space H, and let f(z) be a function defined on a domain D. The spectral function f(T) is defined as the function that maps the operator T to the value of f(z) at the eigenvalues of T. The spectral function is analytic at every point in the spectrum of T, and it can be analytically continued to a larger domain D'.

Conclusion

In conclusion, the concept of analytic continuation for functions of operators is a fundamental tool in understanding the behavior of these functions, particularly in the study of spectral properties of operators. The conditions under which a function of an operator can be analytically continued are:

Spectral properties of the operator: The function f(z) must be analytic at every point in the spectrum of the operator T.
Hilbert-Schmidt property: The operator T must be a Hilbert-Schmidt operator.
Boundedness of the operator: The operator T must be bounded.

If these conditions are satisfied, then the function f(z) can be analytically continued to a larger domain D', where the extended function agrees with the original function on the original domain D.

References

Hilbert, D. (1904). "Grundzüge einer allgemeinen Theorie der linearen Integralgleichungen." Teubner, Leipzig.
Schwartz, L. (1966). "Mathematics for the Physical Sciences." Addison-Wesley, Reading, MA.
Reed, M., & Simon, B. (1972). "Methods of Modern Mathematical Physics." Academic Press, New York.

Introduction

In our previous article, we explored the concept of analytic continuation for functions of operators, including the conditions under which a function of an operator can be analytically continued. In this article, we will address some of the most frequently asked questions related to this topic.

Q: What is the difference between analytic continuation and analytic extension?

A: Analytic continuation and analytic extension are related but distinct concepts. Analytic continuation refers to the process of extending a function from a smaller domain to a larger domain, where the extended function agrees with the original function on the original domain. Analytic extension, on the other hand, refers to the process of extending a function from a smaller domain to a larger domain, without necessarily agreeing with the original function on the original domain.

Q: Can a function of an operator be analytically continued if the operator is unbounded?

A: In general, a function of an operator cannot be analytically continued if the operator is unbounded. This is because the concept of analytic continuation relies on the existence of a power series expansion of the function at a point in the spectrum of the operator. If the operator is unbounded, the power series expansion may not exist, and therefore, the function cannot be analytically continued.

Q: What is the role of the Hilbert-Schmidt property in analytic continuation?

A: The Hilbert-Schmidt property plays a crucial role in analytic continuation. A Hilbert-Schmidt operator is an operator that can be represented as a sum of a countable number of rank-one operators. The Hilbert-Schmidt property ensures that the operator has a finite-dimensional range, which is a necessary condition for analytic continuation.

Q: Can a function of an operator be analytically continued if the operator has a non-trivial kernel?

A: In general, a function of an operator cannot be analytically continued if the operator has a non-trivial kernel. This is because the kernel of the operator represents the set of all vectors that are mapped to zero by the operator. If the kernel is non-trivial, the operator may not have a well-defined inverse, and therefore, the function cannot be analytically continued.

Q: What is the relationship between analytic continuation and the spectral properties of operators?

A: The spectral properties of operators play a crucial role in analytic continuation. The spectrum of an operator represents the set of all complex numbers that are eigenvalues of the operator. If the function is analytic at every point in the spectrum of the operator, then the function can be analytically continued to a larger domain.

Q: Can a function of an operator be analytically continued if the operator has a non-compact resolvent?

A: In general, a function of an operator cannot be analytically continued if the operator has a non-compact resolvent. This is because the resolvent of an operator represents the inverse of the operator minus a complex number. If the resolvent is non-compact, the operator may not have a well-defined inverse, and therefore, the function cannot be analytically continued.

Q: What is the significance of the boundedness of the operator in analytic continuation?

A: The boundedness of the operator is a necessary condition for analytic continuation. If the operator is unbounded, the function may not have a well-defined power series expansion, and therefore, the function cannot be analytically continued.

Conclusion

In conclusion, the concept of analytic continuation for functions of operators is a complex and nuanced topic. The conditions under which a function of an operator can be analytically continued are:

Spectral properties of the operator: The function f(z) must be analytic at every point in the spectrum of the operator T.
Hilbert-Schmidt property: The operator T must be a Hilbert-Schmidt operator.
Boundedness of the operator: The operator T must be bounded.

If these conditions are satisfied, then the function f(z) can be analytically continued to a larger domain D', where the extended function agrees with the original function on the original domain D.

References

Hilbert, D. (1904). "Grundzüge einer allgemeinen Theorie der linearen Integralgleichungen." Teubner, Leipzig.
Schwartz, L. (1966). "Mathematics for the Physical Sciences." Addison-Wesley, Reading, MA.
Reed, M., & Simon, B. (1972). "Methods of Modern Mathematical Physics." Academic Press, New York.