The Monge-Kantorovich transportation problem involves optimizing with respect
to a given a cost function. Uniqueness is a fundamental open question about
which little is known when the cost function is smooth and the landscapes
containing the goods to be transported possess (non-trivial) topology. This
question turns out to be closely linked to a delicate problem (# 111) of
Birkhoff [14]: give a necessary and sufficient condition on the support of a
joint probability to guarantee extremality among all measures which share its
marginals.
The purpose of this note is to show that the solution to the Kantorovich
optimal transportation problem is supported on a Lipschitz manifold, provided
the cost is $C^{2}$ with non-singular mixed second derivative. We use this
result to provide a simple proof that solutions to Monge's optimal
transportation problem satisfy a change of variables equation almost
everywhere.
A principal wishes to transact business with a multidimensional distribution
of agents whose preferences are known only in the aggregate. Assuming a twist
(= generalized Spence-Mirrlees single-crossing) hypothesis and that agents can
choose only pure strategies, we identify a structural condition on the
preference b(x,y) of agent type x for product type y -- and on the principal's
costs c(y) -- which is necessary and sufficient for reducing the profit
maximization problem faced by the principal to a convex program.
Consider transportation of one distribution of mass onto another, chosen to
optimize the total expected cost, where cost per unit mass transported from x
to y is given by a smooth function c(x,y).
The Ma-Trudinger-Wang curvature -- or cross-curvature -- is an object arising
in the regularity theory of optimal transportation. We study this quantity for
costs defined by natural mechanical actions. We show the least action
corresponding to a harmonic oscillator has zero cross-curvature, and in
particular satisfies the necessary and sufficient condition (A3w) for the
continuity of optimal maps.
The Ma-Trudinger-Wang curvature -- or cross-curvature -- is an object arising
in the regularity theory of optimal transportation. We study this quantity for
costs defined by natural mechanical actions. We show the least action
corresponding to a harmonic oscillator has zero cross-curvature, and in
particular satisfies the necessary and sufficient condition (A3w) for the
continuity of optimal maps.