Abstract
RELATIONSHIPS AMONGST THREE FUNCTIONS, WHICH MAY BE DEFINED IN TERMS OF THE PARAMETER ALPHA OF THE SINGLE PARAMETER GRAVITY MODEL, ARE DISCUSSED. THE FUNCTIONS ARE THE ENTROPY H (WHICH IS INTERPRETED AS A MEASURE OF THE INTERACTIVITY IN THE TRANSPORTATION SYSTEM), THE ACCESSIBILITY F (A GENERALIZATION OF HANSEN'S ACCESSIBILITY INDEX), AND C (THE MEAN TRIP COST). IT IS SHOWN THAT THE DERIVATIVE OF H WITH RESPECT TO C EXISTS AND IS EQUAL TO ALPHA, AND THE EXISTENCE IS ESTABLISHED OF A ONE-TO-ONE CORRESPONDENCE BETWEEN F AND H.(Author/publisher).