a formula to calculate the likely interaction between two places given their distance apart, size and barriers (or lack of connectivity) between them.
Iij = Pi x Pj / (dij)k
where I - is the interaction, P - is population, d - is distance and k - is the resistance to movement. i and j are the two places. Distance could be substituted by time or cost where appropriate.
Geography Dictionary & Glossary for Students \\ ITS Tutorial School (ITS) - Hong Kong.