\begin{equation}
	\hat{H} = \frac{\alpha + \beta}{4} \, \sum_{k=0}^{\pi/2}
	\, \cos k \,\, \vec{v}_k^\dagger \, \left(
	\begin{array}{rrrr}
	1 &  0 &  0 & 1 \\
	0 & -1 & -1 & 0 \\
	0 & -1 & -1 & 0 \\
	1 &  0 &  0 & 1
	\end{array}
	\right ) \, \vec{v}_k \, , \quad \vec{v}_k = \left(
	\begin{array}{l}
	\hat{a}_k \\
	\hat{a}_{-k}^\dagger \\
	\hat{a}_{k-\pi} \\
	\hat{a}_{-(k-\pi)}^\dagger
	\end{array} \right )
\end{equation}

\begin{eqnarray}
	B(t) \leq \frac{2}{\chi} \, \int \limits_{0}^t \! \mathrm{d}t_1
	\int \limits_{0}^{t_1} \! \mathrm{d}t_2 \int \limits_{0}^{t_2} \!
	\mathrm{d}t_3 \!\!\!\! && \!\!\!\! \Big [ \langle \underline{J}(0)
	\underline{J}(t_1) \underline{J}(t_2) \underline{J}(t_3)
	\rangle_\mathrm{Gleichgewicht} \nonumber \\[-0.25cm]
	\!\!\!\! && \!\!\!\! - f'(t_1) g'(t_2-t_3) - f''(t_2) g(t_1-t_3)
	- f(t_3) g''(t_1-t_2) \Big ]
\end{eqnarray}

$\partial \varepsilon_{1,2}/ \partial \lambda \approx \pm \hbar
\sqrt{\mu^2 + \nu^2 + 2 \, \mu \nu \exp (2 \lambda)}$