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20180620 22:33:54 +0200  asked a question  How to substitute values for derivatives? I would like to first differentiate a function, then substitute values for derivatives within the function. For example, a function might be I believe the error comes from trying to define the entire function theta(t), diff(phi(t),t,t), etc. as a single number. It should work just as well to substitute other variables in the place of the derivatives; I only want the derivatives there for getting the initial equations. Is there a way to either:
