I am looking for parametric production functions that do not satisfy weak homothetic separability (as first defined in Shephard, 1953), but that do allow for an analytical expression of the dual cost function to be derived from the cost minimization problem (they are self-dual). Do any exist?
For completeness, one can say a production function with variable inputs $M_{it}$, $L_{it}$ and fixed input $K_{it}$ is weakly homothetically separable if it is of the form,
\begin{equation} q_{it} = F\left( K_{it}, h( K_{it}, L_{it}, M_{it} ) \right), \end{equation}
with $h( K_{it}, \cdot, \cdot )$ homogeneous of arbitrary degree for all $K_{it}$.
References
Shephard, Ronald W. COST AND PRODUCTION FUNCTIONS. PRINCETON UNIV NJ, 1953.