  The single-mode spin-boson model (SMSBM) has extensive application in
different subfields of physics. In the absence of rotating-wave approximation
(RWA), we try to solve SMSBM analytically. We argue that the analytical
expression obtained is the most exact approximation to the solution of the
system under the assumption of Abel-Ruffini theorem, which works for a wide
range of the parameters such as coupling strength and detuning and would be
practical for currently available experiments.
