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\[ \Delta_0^1 A = \int_{\lambda=0}^{\lambda=1} \left< \frac{\partial
    U(\lambda)}{\partial\lambda} \right>_\lambda \mathrm{d}\lambda
\approx \sum_{i=0}^{n-1} w_i \left< \frac{U(\lambda_{i} + \delta) -
    U(\lambda_i)}{\delta} \right>_{\lambda_i} \]

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