In these slides, specifically page 11, the following formula is reported:
$$ \frac{1}{N} \sum_{k=1}^N \frac{L_i(l_k)f(l_k,v)\cos(\theta_{l_k})}{p(l_k,v)} \approx \left( \frac{1}{N} \sum_{k=1}^N L_i(l_k) \right) \left( \frac{1}{N} \sum_{k=1}^N \frac{f(l_k,v)\cos(\theta_{l_k})}{p(l_k,v)} \right) $$
The formula above should be an approximation of the rendering equations. I'm looking for a paper that explains how that formula is derived, I cannot manage to find neither the title of a paper (other than the name "Dimitar" mentioned in the same page I pointed out).
I've done some research about some possible method that would justify. This is the closest thing I managed to find, however there's some hypothesis that doesn't seem to me fits.