  What does it mean to study PDE(=Partial Differential Equation)? How and what
to do "to claim proudly that I'm studying a certain PDE"? Newton mechanic uses
mainly ODE(=Ordinary Differential Equation) and describes nicely movements of
Sun, Moon and Earth etc. Now, so-called quantum phenomenum is described by, say
Schr\"odinger equation, PDE which explains both wave and particle characters
after quantization of ODE. The coupled Maxwell-Dirac equation is also
"quantized" and QED(=Quantum Electro-Dynamics) theory is invented by
physicists. Though it is said this QED gives very good coincidence between
theoretical and experimental observed quantities, but what is the equation
corresponding to QED? Or, is it possible to describe QED by "equation" in naive
sense?
