The pdetest command returns either 0 (when the PDE is annulled by the solution sol), indicating that the solution is correct, or a remaining algebraic expression (obtained after simplifying the PDE with respect to the proposed solution), indicating that the solution might be wrong.

When PDE is a system, given as a set or list, possibly including boundary conditions, for each of the elements in the set/list pdetest will return a 0 or the remaining algebraic expression; the advantage of giving PDE as a list is that you can thus determine which element (if any) is not satisfied by the solution.

The pdetest command can also be used to reduce a PDE to a simpler problem by giving an "ansatz", instead of an explicit solution, since it will return the nonzero remaining part.