E_{0 Σ}^{2} = E_{0 k}^{2} {\sum_{k} \exp (i φ_{k}) \exp (- i φ_{k}) + \sum_{l} \exp (i φ_{l}) \sum_{m \neq l} \exp (- i φ_{m})}

${E_{0 \Sigma}^2 = E_{0k}^2 \{\sum_k \exp(i \varphi_k) \exp( - i \varphi_k) + \sum_l \exp(i \varphi_l) \sum_{m \neq l} \exp(-i \varphi_m )\}}$

Error