= Variational principle
A variational principle specifies a <function>, path or field by requiring a <functional> to be stationary under an allowed class of <variations>. The admissible class includes the prescribed <boundary conditions> and <constraints>. A stationary value need not be a <minimum>: the <Euler-Lagrange equation> expresses the first-order stationarity condition, while a minimum requires additional analysis. <Fermat principle> applies this idea to optical path length, and the <Rayleigh-Ritz method> approximates a variational problem in a chosen family of <trial functions>.
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