In differential equations, the Laplace invariant of any of certain differential operators is a certain function of the coefficients and their derivatives. Consider a bivariate hyperbolic differential operator of the second order
whose coefficients
are smooth functions of two variables. Its Laplace invariants have the form
Their importance is due to the classical theorem:
Theorem: Two operators of the form are equivalent under gauge transformations if and only if their Laplace invariants coincide pairwise.
Here the operators
are called equivalent if there is a gauge transformation that takes one to the other:
Laplace invariants can be regarded as factorization "remainders" for the initial operator A:
If at least one of Laplace invariants is not equal to zero, i.e.
then this representation is a first step of the Laplace–Darboux transformations used for solving non-factorizable bivariate linear partial differential equations (LPDEs).
If both Laplace invariants are equal to zero, i.e.
then the differential operator A is factorizable and corresponding linear partial differential equation of second order is solvable.
Laplace invariants have been introduced for a bivariate linear partial differential operator (LPDO) of order 2 and of hyperbolic type. They are a particular case of generalized invariants which can be constructed for a bivariate LPDO of arbitrary order and arbitrary type; see Invariant factorization of LPDOs.