= Landau approximation
{c}
{title2=$F_{\rm L}[h]=H[m_h,h]$}
The Landau approximation evaluates a statistical field's <partition function> at a stable stationary configuration, omitting fluctuation-loop corrections to the <free energy>. Functional differentiation of that stationary value still gives a connected linear response: the inverse of the saddle's quadratic fluctuation operator. Freezing fields before differentiating would lose this response. A <Gaussian field theory>, in contrast, permits exact integration of its quadratic fluctuations.
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