Spectral analysis of non-commutative harmonic oscillators: the lowest eigenvalue and no crossing

The lowest eigenvalue of non-commutative harmonic oscillators $Q$ is studied. It is shown that $Q$ can be decomposed into four self-adjoint operators, and all the eigenvalues of each operator are simple. We show that the lowest eigenvalue $E$ of $Q$ is simple. Furthermore a Jacobi matrix representation of $Q$ is given and spectrum of $Q$ is considered numerically.