  We study competitive equilibria in the classic Shapley-Shubik assignment
model with indivisible goods and unit-demand buyers, with budget constraints:
buyers can specify a maximum price they are willing to pay for each item,
beyond which they cannot afford the item. This single discontinuity introduced
by the budget constraint fundamentally changes the properties of equilibria: in
the assignment model without budget constraints, a competitive equilibrium
always exists, and corresponds exactly to a stable matching. With budgets, a
competitive equilibrium need not always exist. In addition, there are now two
distinct notions of stability, depending on whether both or only one of the
buyer and seller can strictly benefit in a blocking pair, that no longer
coincide due to the budget-induced discontinuity. We define weak and strong
stability for the assignment model with transferable utilities, and show that
competitive equilibria correspond exactly to strongly stable matchings.
  We consider the algorithmic question of efficiently computing competitive
equilibria in an extension of the assignment model with budgets, where each
buyer specifies his preferences over items using utility functions $u_{ij}$,
where $u_{ij}(p_j)$ is the utility of buyer $i$ for item $j$ when its price is
$p_j$. Our main result is a strongly polynomial time algorithm that decides
whether or not a competitive equilibrium exists and if yes, computes a minimum
one, for a general class of utility functions $u_{ij}$. This class of utility
functions includes the standard quasi-linear utility model with a budget
constraint, and in addition, allows modeling marketplaces where, for example,
buyers only have a preference ranking amongst items subject to a maximum
payment limit for each item, or where buyers want to optimize return on
investment (ROI) instead of a quasi-linear utility and only know items'
relative values.
