  The true level crossing in the asymmetric quantum Rabi model without any
obvious symmetry can be exhibited in the energy spectrum if the qubit bias is a
multiple of the cavity frequency, which should imply the existence of the
hidden symmetry. In this work, within a Bogoliubov operator approach, we can
readily derive the symmetry operators associated with the hidden symmetry
hierarchically for arbitrary multiples. The symmetry operators for small
multiples in the literature can be extremely easily reproduced in our general
scheme. In addition, a general parity operator is defined through the symmetry
operator, which naturally includes the well-known parity operator of the
symmetric model. We believe that the present approach can be straightforwardly
extended to other asymmetric Rabi models to find the relevant symmetry
operators.
