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.
I took three semesters of calculus, and one of advanced calculus. I never
understood anything, really, until the next semester. The stuff in advanced
calculus is still over my head.
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