User:Holmansf/Egorov's theorem (pseudodifferential operators)

In the theory of pseudodifferential operators (ψDOs), an area of mathematics, Egorov's Theorem shows that when a ψDO is conjugated by Fourier integral operators, the result is again a ψDO. Furthermore, the theorem gives a simple formula for the principal symbol of the resulting ψDO. The theorem is named after Yurii Vladimirovich Egorov (see here for a brief profile) who first proved the result in. The theorem is a major ingredient in the parametrix construction and resulting existence results for general partial differential equations of principal type.