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Why D[x', x] isn't 0 ?
Hello! D[x'', x'] => 0 (* okay *) D[x', x] => x'' (* unexpected despite being similar to above *) D[f[x'], x] => x'' f'[x'] (* unexpected as well *) Why the last 2 lines don't produce zeroes and how to make them do so (for any function f) because it's quite common in mechanics where velocity is treated as independent of position, etc. -- Vladimir