Solution

ID: past-exam-of-the-theoretical-physics-ii-course-of-the-university-of-cambridge/2014/paper-1/2/a/solution

The quantum propagator is the position-space kernel of the time-evolution operator: . For a time-independent Quantum Hamiltonian, . Inserting a complete orthonormal energy eigenbasis gives
A continuous spectrum requires the corresponding integral. At equal times, completeness makes this kernel a Dirac delta function.

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