Discounting terminology confusion
Well this is 'exponential discounting'. An infinite sum of geometric series:
$$\sum R+ R\delta + R\delta^2.... R\delta^t = \frac{R}{1-\delta}, \text{ if } |\delta|&lt;1$$
Now lets call the denominator rho $1-\delta= \rho$.
The exponential discounting is there since its an infinite sum of geometric series.
Edit: In response to Giskard's +1 comment I tried to look deeper into the paper and while I think that the Dixit is actually implying the above and just not being clear on terminology it is also possible that he means simply net present value of infinite constant stream as:
$$\sum \frac{R}{(1+\rho)^t}= \frac{R}{\rho}$$
However, I think the former interpretation is the right one because its more closer to how discount rates are treated in some other papers I have seen and I think he was just sloppy with terminology.