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He then states that if we expand the square root in the RHS in powers of ##\nabla^2##, then we get an infinite number of spatial derivatives acting on ##\psi(x,t)## which implies that the above equation is non-local in space. I don't get why this implication follows (i.e. why an infinite number of spatial derivatives acting on the position-space wavefunction implies that the equation is non-local in space), could someone explain it? Thanks!