@article{iiasa14147,
          volume = {4},
          number = {5},
           title = {Mixed qualitative calculus and economic modeling},
       publisher = {Gordon and Breach Science Publishers, Inc},
            year = {1987},
           pages = {399--408},
         journal = {Systems Analysis Modelling Simulation},
             url = {http://hdl.handle.net/11370/340c68dc-6e82-4ccf-86cc-f35c0db7cd85},
            issn = {0232-9298},
          author = {Brouwer, F. and Nijkamp, P. and Voogd, H.}
}