The gravity equation, black market exchange rates, market size, and Linder's hypothesis / Bruno Larue, Joshua Mutunga. -- Guelph : University of Guelph, 1991. -- 19 s. : 3 tab. -- (Working Papers Series 91/14. Department of Agric. econ. and business). obchod čierny * obchod poľnohospodársky