Solution
= Solution
Define $|\pm\rangle_1=(|0\rangle_1\pm|1\rangle_1)/\sqrt2$. Since $B_1+B_1^\dagger$ is the Pauli $X$ operator on the first oscillator, the exact <eigenvectors> are
$$
|\pm\rangle_1\otimes|n_2\rangle_2,
$$
with energies
$$
E_{\pm,n_2}=\pm g+E_2n_2,\qquad n_2=0,1.
$$