of the flows
of bilateral trade
based on analogy with the law of gravity in physics: Tij
, where Tij
from country i to country j, Yi
are their national incomes
is the distance between them, and A is a constant. Other constants as exponents and other variables
are often included. Due independently to Tinbergen (1962) and PÃ¶yhÃ¶nen (1963).