A functional differential equation is, in one common usage, a differential equation in which the derivative 
of an unknown function 
has a value at 
that is related to 
as a function of some other function at 
. A general first-order functional differential equation is
therefore given by
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In mathematical physics, the term is also used for an equation whose unknown is a functional and which contains one or more functional derivatives. The Dyson-Schwinger equations and the Wetterich equation are examples.
