is: an equation in which an operator acts on an eigenfunction to produce the eigenfunction, multiplied by an eigenvalue. That is, for an operator
where 
is an eigenfunction of and 
is the eigenvalue of 
belonging to the particular eigenfunction. [P10.4, P11.1, P11.2, P11.3]
permits: more than one (and possibly an infinite number) of eigenvalues and eigenfunctions for a given operator. In physical problems is most commonly a differential operator, but it can take other forms. In quantum physics, is commonly a spatial wavefunction (i.e. an eigenfunction of the energy operator, the Hamiltonian). [P10.4, P11.1, P11.2, P11.3]
Copyright 1997, The Open University