Edward Ullman

Multiple nuclei model by Chauncy Harris and Edward Ullman

Edward Ullman (1912 – 1976) was an American geographer. He proposed that trade was an interaction based on three phenomena: complementarity, intervening opportunities, and transferrability of commodities.

The level of interaction can be measured by the Gravity model of trade:

$I_{i,j} = \frac{p_ip_j}{d_{i,j}^\beta}$

Where:

• I: Level of interaction between i,j. alternatively, quantity of trade between i,j.
• P_i = population of i
• D_i,j = distance separating i,j
• β = impedance factor

External links

• University of Washington