1. Sturm–Liouville Theory

Under the assumptions that the S–L problem is regular, that is, p(x)⁻¹ > 0, q(x), and w(x) > 0 are real-valued integrable functions over the finite interval [a, b], with separated boundary conditions of the form

[math]\displaystyle{ y(a)\cos \alpha - p(a)y^\prime (a)\sin \alpha = 0, }[/math]

(1)

[math]\displaystyle{ y(b)\cos \beta - p(b)y^\prime (b)\sin \beta = 0, }[/math]

(2)

where [math]\displaystyle{ \alpha, \beta \in [0, \pi), }[/math]

the main tenet of Sturm–Liouville theory states that:

• The eigenvalues λ₁, λ₂, λ₃, ... of the regular Sturm–Liouville problem (1)-(2)-(3) are real and can be ordered such that

[math]\displaystyle{ \lambda_1 \lt \lambda_2 \lt \lambda_3 \lt \cdots \lt \lambda_n \lt \cdots \to \infty; \, }[/math]

• Corresponding to each eigenvalue λ_n is a unique (up to a normalization constant) eigenfunction y_n(x) which has exactly n − 1 zeros in (a, b). The eigenfunction y_n(x) is called the n-th fundamental solution satisfying the regular Sturm–Liouville problem (1)-(2)-(3).

• The normalized eigenfunctions form an orthonormal basis

[math]\displaystyle{ \int_a^b y_n(x)y_m(x)w(x)\,dx = \delta_{mn}, }[/math]

in the Hilbert space L²([a, b],w(x) dx). Here δ_mn is a Kronecker delta.

Note that, unless p(x) is continuously differentiable and q(x), w(x) are continuous, the equation has to be understood in a weak sense.

2. Sturm–Liouville Form

The differential equation (1) is said to be in Sturm–Liouville form or self-adjoint form. All second-order linear ordinary differential equations can be recast in the form on the left-hand side of (1) by multiplying both sides of the equation by an appropriate integrating factor (although the same is not true of second-order partial differential equations, or if y is a vector.)

2.1. Examples

The Bessel equation:

[math]\displaystyle{ x^2y''+xy'+(\lambda^2x^2-\nu^2)y=0\, }[/math]

can be written in Sturm–Liouville form as

[math]\displaystyle{ (xy')'+(\lambda^2 x-\nu^2/x)y=0.\, }[/math]

The Legendre equation,

[math]\displaystyle{ (1-x^2)y''-2xy'+\nu(\nu+1)y=0\;\! }[/math]

can easily be put into Sturm–Liouville form, since D(1 − x²) = −2x, so, the Legendre equation is equivalent to

[math]\displaystyle{ [(1-x^2)y']'+\nu(\nu+1)y=0\;\! }[/math]

Less simple is such a differential equation as

[math]\displaystyle{ x^3y''-xy'+2y=0.\, }[/math]

Divide throughout by x³:

[math]\displaystyle{ y''-{x\over x^3}y'+{2\over x^3}y=0 }[/math]

Multiplying throughout by an integrating factor of

[math]\displaystyle{ e^{\int -{x / x^3}\,dx}=e^{\int -{1 / x^2}\, dx}=e^{1 / x}, }[/math]

gives

[math]\displaystyle{ e^{1 / x}y''-{e^{1 / x} \over x^2} y'+ {2 e^{1 / x} \over x^3} y = 0 }[/math]

which can be easily put into Sturm–Liouville form since

[math]\displaystyle{ D e^{1 / x} = -{e^{1 / x} \over x^2} }[/math]

so the differential equation is equivalent to

[math]\displaystyle{ (e^{1 / x}y')'+{2 e^{1 / x} \over x^3} y =0. }[/math]

In general, given a differential equation

[math]\displaystyle{ P(x)y''+Q(x)y'+R(x)y=0\, }[/math]

dividing by P(x), multiplying through by the integrating factor

[math]\displaystyle{ e^{\int {Q(x) / P(x)}\,dx}, }[/math]

and then collecting gives the Sturm–Liouville form.

3. Sturm–Liouville Equations as Self-adjoint Differential Operators

The map

[math]\displaystyle{ L u = {1 \over w(x)} \left(-{d\over dx}\left[p(x){du\over dx}\right]+q(x)u \right) }[/math]

can be viewed as a linear operator mapping a function u to another function Lu. We may study this linear operator in the context of functional analysis. In fact, equation (1) can be written as

[math]\displaystyle{ L u = \lambda u \,. }[/math]

This is precisely the eigenvalue problem; that is, we are trying to find the eigenvalues λ₁, λ₂, λ₃, ... and the corresponding eigenvectors u₁, u₂, u₃, ... of the L operator. The proper setting for this problem is the Hilbert space L²([a, b],w(x) dx) with scalar product

[math]\displaystyle{ \langle f, g\rangle = \int_{a}^{b} \overline{f(x)} g(x)w(x)\,dx. }[/math]

In this space L is defined on sufficiently smooth functions which satisfy the above boundary conditions. Moreover, L gives rise to a self-adjoint operator. This can be seen formally by using integration by parts twice, where the boundary terms vanish by virtue of the boundary conditions. It then follows that the eigenvalues of a Sturm–Liouville operator are real and that eigenfunctions of L corresponding to different eigenvalues are orthogonal. However, this operator is unbounded and hence existence of an orthonormal basis of eigenfunctions is not evident. To overcome this problem one looks at the resolvent

[math]\displaystyle{ (L - z)^{-1}, \qquad z \in\mathbb{C}, }[/math]

where z is chosen to be some real number which is not an eigenvalue. Then, computing the resolvent amounts to solving the inhomogeneous equation, which can be done using the variation of parameters formula. This shows that the resolvent is an integral operator with a continuous symmetric kernel (the Green's function of the problem). As a consequence of the Arzelà–Ascoli theorem this integral operator is compact and existence of a sequence of eigenvalues α_n which converge to 0 and eigenfunctions which form an orthonormal basis follows from the spectral theorem for compact operators. Finally, note that [math]\displaystyle{ (L-z)^{-1} u = \alpha u }[/math] is equivalent to [math]\displaystyle{ L u = (z+\alpha^{-1}) u }[/math].

If the interval is unbounded, or if the coefficients have singularities at the boundary points, one calls L singular. In this case the spectrum does no longer consist of eigenvalues alone and can contain a continuous component. There is still an associated eigenfunction expansion (similar to Fourier series versus Fourier transform). This is important in quantum mechanics, since the one-dimensional Schrödinger equation is a special case of a S–L equation.

4. Example

We wish to find a function u(x) which solves the following Sturm–Liouville problem:

[math]\displaystyle{ L u = \frac{d^2u}{dx^2} = \lambda u }[/math]

(3)

where the unknowns are λ and u(x). As above, we must add boundary conditions, we take for example

[math]\displaystyle{ u(0) = u(\pi) = 0 \, }[/math]

Observe that if k is any integer, then the function

[math]\displaystyle{ u(x) = \sin kx \, }[/math]

is a solution with eigenvalue λ = −k². We know that the solutions of a S–L problem form an orthogonal basis, and we know from Fourier series that this set of sinusoidal functions is an orthogonal basis. Since orthogonal bases are always maximal (by definition) we conclude that the S–L problem in this case has no other eigenvectors.

Given the preceding, let us now solve the inhomogeneous problem

[math]\displaystyle{ L u =x, \qquad x\in(0,\pi) }[/math]

with the same boundary conditions. In this case, we must write f(x) = x in a Fourier series. The reader may check, either by integrating ∫exp(ikx)x dx or by consulting a table of Fourier transforms, that we thus obtain

[math]\displaystyle{ L u =\sum_{k=1}^{\infty}-2\frac{(-1)^k}{k}\sin kx. }[/math]

This particular Fourier series is troublesome because of its poor convergence properties. It is not clear a priori whether the series converges pointwise. Because of Fourier analysis, since the Fourier coefficients are "square-summable", the Fourier series converges in L² which is all we need for this particular theory to function. We mention for the interested reader that in this case we may rely on a result which says that Fourier's series converges at every point of differentiability, and at jump points (the function x, considered as a periodic function, has a jump at π) converges to the average of the left and right limits (see convergence of Fourier series).

Therefore, by using formula (4), we obtain that the solution is

[math]\displaystyle{ u=\sum_{k=1}^{\infty}2\frac{(-1)^k}{k^3}\sin kx. }[/math]

In this case, we could have found the answer using antidifferentiation. This technique yields u = (x³ − π²x)/6, whose Fourier series agrees with the solution we found. The antidifferentiation technique is no longer useful in most cases when the differential equation is in many variables.

5. Application to Normal Modes

Suppose we are interested in the modes of vibration of a thin membrane, held in a rectangular frame, 0 < x < L₁, 0 < y < L₂. We know the equation of motion for the vertical membrane's displacement, W(x, y, t) is given by the wave equation:

[math]\displaystyle{ \frac{\partial^2W}{\partial x^2}+\frac{\partial^2W}{\partial y^2} = \frac{1}{c^2}\frac{\partial^2W}{\partial t^2}. }[/math]

The equation is separable (substituting W = X(x) × Y(y) × T(t)), and the normal mode solutions that have harmonic time dependence and satisfy the boundary conditions W = 0 at x = 0, L₁ and y = 0, L₂ are given by

[math]\displaystyle{ W_{mn}(x,y,t) = A_{mn}\sin\left(\frac{m\pi x}{L_1}\right)\sin\left(\frac{n\pi y}{L_2}\right)\cos\left(\omega_{mn}t\right) }[/math]

where m and n are non-zero integers, A_mn is an arbitrary constant and

[math]\displaystyle{ \omega^2_{mn} = c^2 \left(\frac{m^2\pi^2}{L_1^2}+\frac{n^2\pi^2}{L_2^2}\right). }[/math]

Since the eigenfunctions W_mn form a basis, an arbitrary initial displacement can be decomposed into a sum of these modes, which each vibrate at their individual frequencies [math]\displaystyle{ \omega_{mn} }[/math]. Infinite sums are also valid, as long as they converge.