I have been reviewing calculations to find walrasian equilibrium and pareto efficient allocations.
Assume we are in an environment with two consumers, $A$ and $B$, and two goods, $x$ and $y$. From this article in wikipedia, we have that a sufficient condition for pareto efficiency is that the marginal rates of substitution are equal for both consumers.
However for practical purposes this seems only useful when utility functions are differentiable. Assuming preferences are continuous and locally non-satiated, is there any "fast and loose" rule for pareto efficiency when utility functions are not differentiable?
For example with leontieff preferences $u(x,y) = \min\{ x,2y \}$ in which case there might be a corner solution.