From the The Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel‘s description of Krugman’s scientific contributions:
Trade and Geography — Economies of Scale, Differentiated Products and Transport Costs
By the late 1980s, researchers had begun to integrate economies of scale into general equilibrium models of location and trade, thereby giving precision to the verbal analyses of earlier researchers and adding important new insights. In the resulting work, now commonly known as the new economic geography, economic geographers made use of the new tools, along with economists who took a renewed interest in the field. Several researchers took part in these developments, but the most influential contributions were made by Paul Krugman.
