While the 1940 physicists used an enormous amount of pompous notation, its clear they were very practical folk – remembering that they had no computers to do numerical work. Everything had to be done by reasoning, where possible. Using the tools of today, its fun to read what they were teaching. two of those tools are recurrence relations (such as that at the heart of DES), and solving partial differential equations formulated to describe the motion of trajectories through state/phase space.
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Peter's ruminations 