An Efficient Multi-Unit Ascending Auction

We provide an ascending auction that yields an efficient outcome when there are many identical units for sale and bidders have interdependent values and downward-sloping demand. Our ascending auction both extends and generalizes Ausubel's (2004) and yields the same outcome as Perry and Reny's (2002) generalization of Vickrey's (1961) sealed-bid auction. There are two key features of our auction. Bidders are permitted both to express different demands against different bidders, as well as to increase their demands. The equilibrium strategies are closely related to the familiar "drop out when price equals value" strategy of the English auction. Copyright 2005, Wiley-Blackwell.