Yes, I saw that but I thought you might be interested in their
slightly different approach. You also get a working script which you
can modify with different assumptions, and automatically produce
graphs.
I'll have to admit I didn't get as deep as differential calculus!
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I studied calculus in engineering school but never considered myself
to be highly proficient at it. A few years later I was on the
technical staff of a high energy physics lab at Cornell University. On
of my colleagues was a PhD EE who could solve differential equations
in his head. That always amazed me.