\int_{-\frac{L}{2}}^{\frac{L}{2}}\int_{f([-\frac{L}{2},\frac{L}{2}])}p(x_0)p(y|x_0)\ln p(y)dydx_0\approx \frac{1}{2L}\int_{-\frac{L}{2}}^{\frac{L}{2}}\ln \left(\frac{f'(x_0)}{\sigma}\right)^2dx_0 | \int_{-\frac{L}{2}}^{\frac{L}{2}}\int_{f([-\frac{L}{2},\frac{L}{2}])}p(x_0)p(y|x_0)\ln p(y)dydx_0\approx \frac{1}{2L}\int_{-\frac{L}{2}}^{\frac{L}{2}}\ln \left(\frac{f'(x_0)}{\sigma}\right)^2dx_0 |