# Fredholm operator

In mathematics, a Fredholm operator is an operator that arises in the Fredholm theory of integral equations. It is named in honour of Erik Ivar Fredholm.

A Fredholm operator is a bounded linear operator T : X → Y between two Banach spaces with finite-dimensional kernel ${\displaystyle \ker T}$ and (algebraic) cokernel ${\displaystyle \mathrm {coker} \,T=Y/\mathrm {ran} \,T}$, and with closed range ${\displaystyle \mathrm {ran} \,T}$. The last condition is actually redundant.[1]

Equivalently, an operator T : X → Y is Fredholm if it is invertible modulo compact operators, i.e., if there exists a bounded linear operator

${\displaystyle S:Y\to X}$

such that

${\displaystyle \mathrm {Id} _{X}-ST\quad {\text{and}}\quad \mathrm {Id} _{Y}-TS}$

are compact operators on X and Y respectively.

The index of a Fredholm operator is

${\displaystyle \mathrm {ind} \,T:=\dim \ker T-\mathrm {codim} \,\mathrm {ran} \,T}$

or in other words,

${\displaystyle \mathrm {ind} \,T:=\dim \ker T-\mathrm {dim} \,\mathrm {coker} \,T;}$

see dimension, kernel, codimension, range, and cokernel.

## Properties

The set of Fredholm operators from X to Y is open in the Banach space L(XY) of bounded linear operators, equipped with the operator norm. More precisely, when T0 is Fredholm from X to Y, there exists ε > 0 such that every T in L(XY) with ||TT0|| < ε is Fredholm, with the same index as that of T0.

When T is Fredholm from X to Y and U Fredholm from Y to Z, then the composition ${\displaystyle U\circ T}$ is Fredholm from X to Z and

${\displaystyle \mathrm {ind} (U\circ T)=\mathrm {ind} (U)+\mathrm {ind} (T).}$

When T is Fredholm, the transpose (or adjoint) operator T ′ is Fredholm from Y ′ to X ′, and ind(T ′) = −ind(T). When X and Y are Hilbert spaces, the same conclusion holds for the Hermitian adjoint T.

When T is Fredholm and K a compact operator, then T + K is Fredholm. The index of T remains unchanged under compact perturbations of T. This follows from the fact that the index i(s) of T + sK is an integer defined for every s in [0, 1], and i(s) is locally constant, hence i(1) = i(0).

Invariance by perturbation is true for larger classes than the class of compact operators. For example, when U is Fredholm and T a strictly singular operator, then T + U is Fredholm with the same index.[2] A bounded linear operator T from X into Y is strictly singular when it fails to be bounded below on any infinite-dimensional subspace. In symbols, an operator ${\displaystyle T\in B(X,Y)}$ is strictly singular if and only if

${\displaystyle \inf\{\|Tx\|:x\in X_{0},\,\|x\|=1\}=0.\,}$

for each infinite-dimensional subspace ${\displaystyle X_{0}}$ of ${\displaystyle X}$. The class of inessential operators, which properly contains the class of strictly singular operators, is the "perturbation class" for Fredholm operators. This means an operator ${\displaystyle T\in B(X,Y)}$ is inessential if and only if T+U is Fredholm for every Fredholm operator ${\displaystyle U\in B(X,Y)}$.

## Examples

Let ${\displaystyle H}$ be a Hilbert space with an orthonormal basis ${\displaystyle \{e_{n}\}}$ indexed by the non negative integers. The (right) shift operator S on H is defined by

${\displaystyle S(e_{n})=e_{n+1},\quad n\geq 0.\,}$

This operator S is injective (actually, isometric) and has a closed range of codimension 1, hence S is Fredholm with ${\displaystyle \mathrm {ind} (S)=-1}$. The powers ${\displaystyle S^{k}}$, ${\displaystyle k\geq 0}$, are Fredholm with index ${\displaystyle -k}$. The adjoint S* is the left shift,

${\displaystyle S^{*}(e_{0})=0,\ \ S^{*}(e_{n})=e_{n-1},\quad n\geq 1.\,}$

The left shift S* is Fredholm with index 1.

If H is the classical Hardy space ${\displaystyle H^{2}(\mathbf {T} )}$ on the unit circle T in the complex plane, then the shift operator with respect to the orthonormal basis of complex exponentials

${\displaystyle e_{n}:\mathrm {e} ^{\mathrm {i} t}\in \mathbf {T} \rightarrow \mathrm {e} ^{\mathrm {i} nt},\quad n\geq 0,\,}$

is the multiplication operator Mφ with the function ${\displaystyle \phi =e_{1}}$. More generally, let φ be a complex continuous function on T that does not vanish on ${\displaystyle \mathbf {T} }$, and let Tφ denote the Toeplitz operator with symbol φ, equal to multiplication by φ followed by the orthogonal projection ${\displaystyle P:L^{2}(\mathbf {T} )\to H^{2}(\mathbf {T} )}$:

${\displaystyle T_{\varphi }:f\in H^{2}(\mathrm {T} )\rightarrow P(f\varphi )\in H^{2}(\mathrm {T} ).\,}$

Then Tφ is a Fredholm operator on ${\displaystyle H^{2}(\mathbf {T} )}$, with index related to the winding number around 0 of the closed path ${\displaystyle t\in [0,2\pi ]\mapsto \phi (e^{it})}$: the index of Tφ, as defined in this article, is the opposite of this winding number.

## Applications

The Atiyah-Singer index theorem gives a topological characterization of the index of certain operators on manifolds.

An elliptic operator can be extended to a Fredholm operator. The use of Fredholm operators in partial differential equations is an abstract form of the parametrix method.

## B-Fredholm operators

For each integer ${\displaystyle n}$, define ${\displaystyle T_{n}}$ to be the restriction of ${\displaystyle T}$ to ${\displaystyle R(T^{n})}$ viewed as a map from ${\displaystyle R(T^{n})}$ into ${\displaystyle R(T^{n})}$ ( in particular ${\displaystyle T_{0}=T}$). If for some integer ${\displaystyle n}$ the space ${\displaystyle R(T^{n})}$ is closed and ${\displaystyle T_{n}}$ is a Fredholm operator,then ${\displaystyle T}$ is called a B-Fredholm operator. The index of a B-Fredholm operator ${\displaystyle T}$ is defined as the index of the Fredholm operator ${\displaystyle T_{n}}$. It is shown that the index is independent of the integer ${\displaystyle n}$. B-Fredholm operators were introduced by M. Berkani in 1999 as a generalization of Fredholm operators.[3]

## Notes

1. ^ Yuri A. Abramovich and Charalambos D. Aliprantis, "An Invitation to Operator Theory", p.156
2. ^ T. Kato, "Perturbation theory for the nullity deficiency and other quantities of linear operators", J. d'Analyse Math. 6 (1958), 273–322.
3. ^ Berkani Mohammed: On a class of quasi-Fredholm operators. Integral Equations and Operator Theory, 34, 2 (1999), 244-249 [1]