Edward Ullman
Encyclopedia
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
geographer
Geographer
A geographer is a scholar whose area of study is geography, the study of Earth's natural environment and human society.Although geographers are historically known as people who make maps, map making is actually the field of study of cartography, a subset of geography...
. He proposed that trade
Trade
Trade is the transfer of ownership of goods and services from one person or entity to another. Trade is sometimes loosely called commerce or financial transaction or barter. A network that allows trade is called a market. The original form of trade was barter, the direct exchange of goods and...
was an interaction based on three phenomena: complementarity
Complementarity
-Mathematics:*Complementary angles, in geometry* Complementarity theory, a concept related to optimization -Physical sciences:* Complementarity , a property of nucleic acid molecules in molecular biology...
, intervening opportunities, and transferrability of commodities.
The level of interaction can be measured by the Gravity model of trade:
Where:
- I: Level of interaction between i,j. alternatively, quantity of trade between i,j.
- Pi = population of i
- Di,j = distance separating i,j
- β = impedance factor