# User Contributed Dictionary

## English

### Etymology

From eigen + “function

### Noun

1. A function φ such that, for a generic linear operator D (e.g. differential one), Dφ = λφ where λ is an eigenvalue of the operator.
2. Any eigenfunction of the Hamiltonian operator, representing a quantum state whose energy level is given by the corresponding eigenvalue.

# Extensive Definition

In mathematics, an eigenfunction of a linear operator, A, defined on some function space is any non-zero function f in that space that returns from the operator exactly as is, except for a multiplicative scaling factor. More precisely, one has
\mathcal A f = \lambda f
for some scalar, λ, the corresponding eigenvalue. The solution of the differential eigenvalue problem also depends upon any boundary conditions required of f. In each case there are only certain eigenvalues \lambda=\lambda_n (n=1,2,3,...) that admit a corresponding solution for f=f_n (with each f_n belonging to the eigenvalue \lambda_n) when combined with the boundary conditions. The existence of eigenvectors is typically the most insightful way to analyze A.
For example, f_k(x) = e^ is an eigenfunction for the differential operator
\mathcal A = \frac - \frac
for any value of k, with a corresponding eigenvalue \lambda = k^2 - k. If boundary conditions are applied to this system (e.g., f=0 at two physical locations in space), then only certain values of k=k_n satisfy the boundary conditions, generating corresponding discrete eigenvalues \lambda_n=k_n^2-k_n.

## Applications

Eigenfunctions play an important role in many branches of physics. An important example is quantum mechanics, where the Schrödinger equation
i \hbar \frac \psi = \mathcal H \psi
has solutions of the form
\psi(t) = \sum_k e^ \phi_k,
where \phi_k are eigenfunctions of the operator \mathcal H with eigenvalues E_k. The fact that only certain eigenvalues E_k with associated eigenfunctions \phi_k satisfy Schrödinger's equation leads to a natural basis for quantum mechanics and the periodic table of the elements, with each E_k an allowable energy state of the system. The success of this equation in explaining the spectral characteristics of hydrogen is considered one of the great triumphs of 20th century physics.
Due to the nature of the Hamiltonian operator \mathcal H, its eigenfunctions are orthogonal functions. This is not necessarily the case for eigenfunctions of other operators (such as the example A mentioned above). Orthogonal functions f_i, i=1, 2, \dots, have the property that
0 = \int f_i^ f_j
where f_i^ is the complex conjugate of f_i
whenever i\neq j, in which case the set \ is said to be linearly independent.