General wave perspectives

Many other waves may be investigated with a second order differential equation, function of time and space position, similar to that of Maxwell used in this Photonics book. In many cases, these equations differ one from another only by some coefficients. Without trying to be exhaustive, a few other waves are cited, for which the results obtained in this book may be easily transferred. Particular attention is on the new concept of a robust zero. A robust eigenfunction zero is robust because its eigenstate cannot be activated by any action applied on it. The path states are robust because they are confined within one-dimensional paths, and when they are degenerate they can turn around defects. Using the simple example of path states within one closed loop, it is possible to conjecture the existence of entangled linear photons. This example may be transposed to other kind of waves.