Maybe it is also okay to say: The commutator identity [ d/dt, f(t) ] = 0 is true if and only if f(t)=f(0), i.e. if and only if f(t) is a constant function. Usually [ d/dt, f(t) ] = f'(t), which recovers the earlier statement ( with "only if" added, is this missing from the essay? ). Aside from the relation to QM maths, there is a reason to write the statement as such: The commutation property [d/dt,c]=0 is used all the time when solving problems in integral and differential calculus. But do we yet need a delta-epsilon proof that [d/dt,0]=0? Arg! ( I am sure this is one of the reasons C. Huneke gave me a B in Vector Calculus so many years ago. ) --Brad On Thu, Jul 11, 2019 at 2:01 PM <bradklee@gmail.com> wrote:

I still have nightmares about delta-epsilon proofs