Laplace invariant
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 ∂ x ∂ y + a ∂ x + b ∂ y + c , {\displaystyle \partial _{x}\,\partial _{y}+a\,\partial _{x}+b\,\partial _{y}+c,\,} whose coefficients a = a ( x , y ) , b = c ( x , y ) , c = c ( x , y ) , {\displaystyle a=a(x,y),\ \ b=c(x,y),\ \ c=c(x,y),} are smooth functions of two variables.