The Density equation

In 1976, Nakatsuji sensei find the Density equation.

It's well known that Cho and Cohen and Frishberg have already found it.

However, Nakatsuji's result is the most important as he showed necessary and sufficient
condition. So the density equation is equivalent to Schroedinger equation. Others didn't
prove the sufficient part.

It seems Erdahl has already noticed about it. Unfortunately he never published it.
It's written in his dispersion relation paper, but he knew a bit before. I don't remember
exactly but RDM news or something like that mentioning dispersion operator.