LPFML: A W3C XML Schema for Linear and Integer Programming Robert Fourer Department of Industrial Engineering and Management Sciences, Northwestern University, Evanston, IL, 4er@iems.nwu.edu Leo Lopes Department of Systems and Industrial Engineering, University of Arizona, Tucson, AZ, leo@sie.arizona.edu Kipp Martin Graduate School of Business, University of Chicago, Chicago, IL, kipp.martin@gsb.uchicago.edu There are numerous modeling systems for generating linear programs and numerous solvers for optimizing them. However, it is often not possible for modelers to com- bine their preferred modeling system with their preferred solver. Current modeling systems use their own proprietary model instance formats that various solvers have been adapted to recognize. The existence of all of these formats suggests that one way to encourage modeling system and solver compatibility is to use a standard rep- resentation of a problem instance. Such a standard must: be simple to manipulate and validate; be able to express instance-specific and vendor-specific information; and promote the integration of optimization software with other software. In this paper we present LPFML, an XML Schema for representing linear pro- gramming (LP) instances. In addition, we provide open source C++ libraries that simplify the exchange of problem instance and solution information between mod- eling systems and solvers. We show how our system is used to enable previously unavailable language-solver connections and how our design improves on the state of the art under three different scenarios relevant to communication between solvers and modeling systems. Key Words: linear programming, information systems, XML History: revised 9/01 1