Mathematical Methods of Physics/Linear Algebra

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The simplest structures on which we can study operations of both "algebra" and "calculus" is the Banach space. The crucial importance of Hilbert Spaces in Physics is due to the fact that the not only are Hilbert Spaces a special case of Banach space, but also because they contain the idea of inner product and the related conjugate-symmetry. (This chapter requires some familiarity with basic measure theory)

Inner Product[edit]

Let be a vector space over (here, stands either for or ). The binary operation is said to define an inner product if and only if,

For all ,

(i) (Conjugate Symmetry):

This implies that as

(ii) (Linearity in first variable):

Conjugate symmetry implies that

(iii) (Positivity): for all

(iv) (Definiteness): if and only if

If an inner product is defined on , we say that is an inner product space.

The complex-conjugate is sometimes denoted as

Hilbert Space[edit]

Observe that the positive-definite nature of the inner product implies that we can define a norm on as ,

If is complete under this norm, we say that is a Hilbert Space.

Thus a Hilbert space is a complete inner product space.


In the examples of Hilbert spaces given below, the underlying field of scalars is the complex numbers C, although similar definitions apply to the case in which the underlying field of scalars is the real numbers R.

Euclidean spaces[edit]

Every finite-dimensional inner product space is also a Hilbert space. For example, Cn with the inner product defined by

where the bar over a complex number denotes its complex conjugate.

Sequence spaces[edit]

Given a set B, the sequence space (commonly pronounced "little ell two") over B is defined by

This space becomes a Hilbert space with the inner product

for all x and y in . B does not have to be a countable set in this definition, although if B is not countable, the resulting Hilbert space is not separable. Every Hilbert space is isomorphic to one of the form for a suitable set B. If B=N, the natural numbers, this space is separable and is simply called .

New Hilbert spaces from old[edit]

Two (or more) Hilbert spaces can be combined to produce another Hilbert space by taking either their direct sum or their tensor product.

Square-integrable functions[edit]

Among examples of Hilbert spaces, the one that holds the most interest for the physicists are the spaces.

Consider to be the set of all functions that are square integrable with respect to a real measure , that is is well-defined.

Define inner product on as

Provided the inner product exists for any pair of functions , we can see that is an inner product space.

The reader may notice an ambiguity here, as need not imply that . To resolve this, we use a different equivalence relation between functions, , and hence, at all points of except for a set of points of measure .

The space is an example of what are called the spaces. It can be shown([1]) that all spaces are complete, and hence, the Lebesgue space, is also complete.

Thus, we have that is a Hilbert space.

Let us identify as and denote the inner product of as

The reader with previous experience in quantum mechanics will be able to recognise this as a formal justification for the dirac notation.