 Solution

Write $D=d/dx$. For rapidly decaying smooth functions, integration by parts gives $D^{\ast }=-D$, while multiplication by the real functions $p$ and $q$ is self-adjoint. Thereforeand henceLikewise $D^2$ and multiplication by $q$ are self-adjoint, so their purely imaginary multiples in $A=3i{D}^2-4iq$ are anti-self-adjoint:

If $L\psi =\lambda \psi$ with $\psi \ne 0$, self-adjointness givesConsequently every eigenvalue of $L$ is real.