## When to use spectrum and eigenvalues in linear algebra?

Easy question about linear operators – in physics (often) the terms spectrum and (set of) eigenvalues of an operator are used interchangeably. I’d like a simple compare and contrast to know the difference according to mathematicians. Many thanks! The other answers adequately address the finite-dimensional case.

**What is the set of all eigenvalues called?**

For a given eigenvalue , the set of all x such that T(x) = is called the -eigenspace. The set of all eigenvalues for a transformation is called its spectrum. When the operator T is described by a matrix A, then we’ll associate the eigenvectors, eigenval- ues, eigenspaces, and spectrum to Aas well.

**Is the spectrum of an operator finite or infinite?**

The spectrum of an operator on a finite-dimensional vector space is precisely the set of eigenvalues. However an operator on an infinite-dimensional space may have additional elements in its spectrum, and may have no eigenvalues.

### Is the spectrum an empty subset of C?

The spectrum is always a non-empty closed subset of C, while the set of eigenvalues does not need to be. Also, the set of eigenvalues can be empty (for example, this is the case for the operator S (e n) = e n + 1).

**How is the spectrum of a linear operator defined?**

Since is a linear operator, the inverse is linear if it exists; and, by the bounded inverse theorem, it is bounded. Therefore, the spectrum consists precisely of those scalars for which is not bijective . The spectrum of a given operator is often denoted , and its complement, the resolvent set, is denoted .

**Which is the complex number in the spectrum of T?**

Specifically, a complex number λ is said to be in the spectrum of a bounded linear operator T if is not invertible, where I is the identity operator. The study of spectra and related properties is known as spectral theory, which has numerous applications, most notably the mathematical formulation of quantum mechanics .

#### Which is an example of an empty point spectrum?

While the spectrum is always non-empty, we can have elements with empty point spectrum. As an example, if X = L2([0, 1]) and M: X → X is the multiplication operator, then σ(M) = [0, 1], and each λ ∈ [0, 1] is part of the continuous spectrum (this is an easy exercise, and you can also find it on this site).

**What is the spectrum of a matrix in mathematics?**

OK, so in mathematics the spectrum of a matrix is the set of its eigenvalues . I know it’s confusing, so let me explain. The spectrum of a matrix is basically a linear map of the matrix. You can imagine it as some kind of finite number line, but with eigenvalues as the numbers.