# Fredholm alternative

In mathematics, the Fredholm alternative, named after Ivar Fredholm, is one of Fredholm's theorems and is a result in Fredholm theory. It may be expressed in several ways, as a theorem of linear algebra, a theorem of integral equations, or as a theorem on Fredholm operators. Part of the result states that a non-zero complex number in the spectrum of a compact operator is an eigenvalue.

## Linear algebra

If V is an n-dimensional vector space and $T:V\to V$ is a linear transformation, then exactly one of the following holds:

1. For each vector v in V there is a vector u in V so that $T(u)=v$ . In other words: T is surjective (and so also bijective, since V is finite-dimensional).
2. $\dim(\ker(T))>0.$ A more elementary formulation, in terms of matrices, is as follows. Given an m×n matrix A and a m×1 column vector b, exactly one of the following must hold:

1. Either: A x = b has a solution x
2. Or: AT y = 0 has a solution y with yTb ≠ 0.

In other words, A x = b has a solution $(\mathbf {b} \in \operatorname {Im} (A))$ if and only if for any y such that AT y = 0, it follows that yTb = 0 $(i.e.,\mathbf {b} \in \ker(A^{T})^{\bot })$ .

## Integral equations

Let $K(x,y)$ be an integral kernel, and consider the homogeneous equation, the Fredholm integral equation,

$\lambda \varphi (x)-\int _{a}^{b}K(x,y)\varphi (y)\,dy=0$ and the inhomogeneous equation

$\lambda \varphi (x)-\int _{a}^{b}K(x,y)\varphi (y)\,dy=f(x).$ The Fredholm alternative is the statement that, for every non-zero fixed complex number $\lambda \in \mathbb {C}$ , either the first equation has a non-trivial solution, or the second equation has a solution for all $f(x)$ .

A sufficient condition for this statement to be true is for $K(x,y)$ to be square integrable on the rectangle $[a,b]\times [a,b]$ (where a and/or b may be minus or plus infinity). The integral operator defined by such a K is called a Hilbert–Schmidt integral operator.

## Functional analysis

Results about Fredholm operators generalize these results to complete normed vector spaces of infinite dimensions; that is, Banach spaces.

The integral equation can be reformulated in terms of operator notation as follows. Write (somewhat informally)

$T=\lambda -K$ to mean
$T(x,y)=\lambda \;\delta (x-y)-K(x,y)$ with $\delta (x-y)$ the Dirac delta function, considered as a distribution, or generalized function, in two variables. Then by convolution, $T$ induces a linear operator acting on a Banach space $V$ of functions $\varphi (x)$ $V\to V$ given by
$\varphi \mapsto \psi$ with $\psi$ given by
$\psi (x)=\int _{a}^{b}T(x,y)\varphi (y)\,dy=\lambda \;\varphi (x)-\int _{a}^{b}K(x,y)\varphi (y)\,dy.$ In this language, the Fredholm alternative for integral equations is seen to be analogous to the Fredholm alternative for finite-dimensional linear algebra.

The operator $K$ given by convolution with an $L^{2}$ kernel, as above, is known as a Hilbert–Schmidt integral operator. Such operators are always compact. More generally, the Fredholm alternative is valid when $K$ is any compact operator. The Fredholm alternative may be restated in the following form: a nonzero $\lambda$ either is an eigenvalue of $K,$ or lies in the domain of the resolvent

$R(\lambda ;K)=(K-\lambda \operatorname {Id} )^{-1}.$ ## Elliptic partial differential equations

The Fredholm alternative can be applied to solving linear elliptic boundary value problems. The basic result is: if the equation and the appropriate Banach spaces have been set up correctly, then either

(1) The homogeneous equation has a nontrivial solution, or
(2) The inhomogeneous equation can be solved uniquely for each choice of data.

The argument goes as follows. A typical simple-to-understand elliptic operator L would be the Laplacian plus some lower order terms. Combined with suitable boundary conditions and expressed on a suitable Banach space X (which encodes both the boundary conditions and the desired regularity of the solution), L becomes an unbounded operator from X to itself, and one attempts to solve

$Lu=f,\qquad u\in \operatorname {dom} (L)\subseteq X,$ where fX is some function serving as data for which we want a solution. The Fredholm alternative, together with the theory of elliptic equations, will enable us to organize the solutions of this equation.

A concrete example would be an elliptic boundary-value problem like

$(*)\qquad Lu:=-\Delta u+h(x)u=f\qquad {\text{in }}\Omega ,$ supplemented with the boundary condition

$(**)\qquad u=0\qquad {\text{on }}\partial \Omega ,$ where Ω ⊆ Rn is a bounded open set with smooth boundary and h(x) is a fixed coefficient function (a potential, in the case of a Schrödinger operator). The function fX is the variable data for which we wish to solve the equation. Here one would take X to be the space L2(Ω) of all square-integrable functions on Ω, and dom(L) is then the Sobolev space W 2,2(Ω) ∩ W1,2
0
(Ω), which amounts to the set of all square-integrable functions on Ω whose weak first and second derivatives exist and are square-integrable, and which satisfy a zero boundary condition on ∂Ω.

If X has been selected correctly (as it has in this example), then for μ0 >> 0 the operator L + μ0 is positive, and then employing elliptic estimates, one can prove that L + μ0 : dom(L) → X is a bijection, and its inverse is a compact, everywhere-defined operator K from X to X, with image equal to dom(L). We fix one such μ0, but its value is not important as it is only a tool.

We may then transform the Fredholm alternative, stated above for compact operators, into a statement about the solvability of the boundary-value problem (*)–(**). The Fredholm alternative, as stated above, asserts:

• For each λR, either λ is an eigenvalue of K, or the operator K − λ is bijective from X to itself.

Let us explore the two alternatives as they play out for the boundary-value problem. Suppose λ ≠ 0. Then either

(A) λ is an eigenvalue of K ⇔ there is a solution h ∈ dom(L) of (L + μ0) h = λ−1h ⇔ –μ0+λ−1 is an eigenvalue of L.

(B) The operator K − λ : X → X is a bijection ⇔ (K − λ) (L + μ0) = Id − λ (L + μ0) : dom(L) → X is a bijection ⇔ L + μ0 − λ−1 : dom(L) → X is a bijection.

Replacing -μ0+λ−1 by λ, and treating the case λ = −μ0 separately, this yields the following Fredholm alternative for an elliptic boundary-value problem:

• For each λR, either the homogeneous equation (L − λ) u = 0 has a nontrivial solution, or the inhomogeneous equation (L − λ) u = f possesses a unique solution u ∈ dom(L) for each given datum fX.

The latter function u solves the boundary-value problem (*)–(**) introduced above. This is the dichotomy that was claimed in (1)–(2) above. By the spectral theorem for compact operators, one also obtains that the set of λ for which the solvability fails is a discrete subset of R (the eigenvalues of L). The eigenvalues’ associated eigenfunctions can be thought of as "resonances" that block the solvability of the equation.