Editing Waveguide (section)

== Mathematical waveguides ==
{{Main|Wave equation}}

Waveguides are interesting objects of study from a strictly mathematical perspective. A waveguide (or tube) is defined as type of boundary condition on the wave equation such that the wave function must be equal to zero on the boundary and that the allowed region is finite in all dimensions but one (an infinitely long cylinder is an example.) A large number of interesting results can be proven from these general conditions. It turns out that any tube with a bulge (where the width of the tube increases) admits at least one bound state that exist inside the mode gaps. The frequencies of all the bound states can be identified by using a pulse short in time.  This can be shown using the variational principles. An interesting result by [[Jeffrey Goldstone]] and [[Robert Jaffe (physicist)|Robert Jaffe]] is that any tube of constant width with a twist, admits a bound state.{{sfn|Goldstone|Jaffe|1992}}