In trying to answer the second question in How to determine the order of an ODE?
I have unsuccessfully tried to use
to filter out the appropiate parts of the differential equations.
In these attempts I have hit upon a behaviour of
I do not understand: In my opinion each of the following two code lines (or at the very least the second line) should return the differential expression itself:
But they do not; each line returns
, where the variable
with respect to which is being differentiated is missing. Why? Is it just me having fundamentally misunderstood something?