RDP 9202: Some Tests of Competition in the Australian Housing Loan Market Equation (5)

\frac{d P (Q)}{d Q} \frac{d Q}{d q_{i}} = \frac{d P (Q)}{d Q} \sum_{j = 1}^{n} \frac{d q_{j}}{d q_{i}} = \frac{d P (Q)}{d Q} (1 + \sum_{j \neq i} \frac{d q_{j}}{d q_{i}}) .