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```math \boldsymbol{\sigma}(\mathbf{F}) = \sum_{n=1}^N \frac{n\mu_n}{J}\left[\mathrm{tr}(\mathbf{B}^* ) - 3\right]^{n-1}\,{\mathbf{B}^*}' + \frac{\kappa}{2}\left(J - \frac{1}{J}\right)\mathbf{1} ```