Recovery of Dirac Equations from Their Solutions
  We deal with quantum field theory in the restriction to external Bose fields. Let $(i\gamma^\mu\partial_\mu - \mathcal{B})\psi=0$ be the Dirac equation. We prove that a non-quantized Bose field $\mathcal{B}$ is a functional of the Dirac field $\psi$, whenever this $\psi$ is strictly canonical. Performing the trivial verification for the $\mathcal{B} := m = $ constant which yields the free Dirac field, we also prepare the tedious verifications for all $\mathcal{B}$ which are non-quantized and static. Such verifications must not be confused, however, with the easy and rigorous proof of our formula, which is shown in detail. 
Kurt Just and Zbigniew Oziewicz and Erwin Sucipto