Solve Nonlinear Systems of PDEs by Order Completion: Can There Be a General Nonlinear PDE Theory for

Contrary to widespread perception, there has ever since 1994 been a unified, general, that is, type independent theory for the existence and regularity of solutions for very large classes of nonlinear systems of PDEs, with possibly associated initial and/or boundary value problems, see [21,22], and for further developments [1-3,47-56,58,64-66]. This solution method is based on the Dedekind order completion of suitable spaces of piece-wise smooth functions on the Euclidean domains of definition of the respective PDEs. All the solutions obtained have a blanket, universal, minimal regularity property, namely, they can be assimilated with usual measurable functions or even with Hausdorff continuous functions on the respective Euclidean domains. It is important to note that the use of the order completion method does not require any monotonicity conditions on the nonlinear systems of PDEs involved. One of the major advantages of the order completion method is that it eliminates the algebra based dichotomy "linear versus nonlinear" PDEs, treating both cases equally. Furthermore, the order completion method does not introduce the dichotomy "monotonous versus non-monotonous" PDEs. None of the known functional analytic methods can exhibit such a powerful general performance, since in addition to topology, such methods are significantly based on algebra, and vector spaces do inevitably differentiate between linear and nonlinear entities. The power of the order completion method is also shown in its ability to solve equations far more general than PDEs, and give in fact necessary and sufficient conditions for the existence of their solutions, as well as explicit expressions for the solutions obtained. In the case of PDEs, another advantage of the order completion method is that in treating initial and/or boundary value problems it avoids the considerable additional difficulties which the usual functional analytic methods encounter. Nevertheless, there are certain basic connections and similarities between the usual functional analytic methods in solving PDEs, and on the other hand, the order completion method. And in fact, the ancient equation x^2 = 2 which had two and a half millennia ago created such a terrible conundrum for Pythagoras, has ultimately been solved by a simple version of the order completion method. Indeed, its irrational solution was obtained in the set R of real numbers, while that set itself was obtained by the Dedekind order completion of the set Q of rational numbers, when that latter set is considered with its natural order relation.