# Linear Operators on Hilbert Spaces

Chapter
Part of the Springer Undergraduate Mathematics Series book series (SUMS)

At the end of Chapter 4 we stated that there is an additional structure on the space of all operators on a Hilbert space which enables us to obtain a simpler characterization of invertibility. This is the “adjoint” of an operator and we start this chapter by showing what this is and giving some examples to show how easy it is to find adjoints. We describe some of the properties of adjoints and show how they are used to give the desired simpler characterization of invertibility. We then use the adjoint to define three important types of operators (normal, self-adjoint and unitary operators) and give properties of these. The set of eigenvalues of a matrix has so many uses in finite-dimensional applications that it is not surprising that its analogue for operators on infinitedimensional spaces is also important. So, for any operator T, we try to find as much as possible about the set {λ ∈ ℂ : T — λI is not invertible} which is called the spectrum of T and which generalizes the set of eigenvalues of a matrix. We conclude the chapter by investigating the properties of those selfadjoint operators whose spectrum lies in [0,∞). Although some of the earlier results in this chapter have analogues for real spaces, when we deal with the spectrum it is necessary to use complex spaces, so for simplicity we shall only consider complex spaces throughout this chapter.