Hi everyone! I have (I think a major) problem so I'm hoping you could give me some useful advice. I am working on my thesis where I am statistically estimating this type of equation: dlny=α+β1*dlnx+β2*dlnz+β3*dlnx*dlnz where dln stands for differencing a natural logarithm of the variable, eg dlnz=ln(zt)-ln(zt-1) so basically the approximation of the growth rate of x. After I estimate this equation, I get certain values for α and for the βs and I have this nice equation. My research question is: What will be the marginal effect of an increase in dlnx on dlny if z doesn't exist (ie it is zero). Obviously, the derivative of dlny with respect to dlnx is ∂lny/∂lnx=β1+β3*dlnz and now I need to calculate how much that is when z (not dlnz) doesn't exist. Obviously, I have problem since dlnz in this case isn't defined: dlnz=ln(0)-ln(0) Is there any way I can manipulate mathematically my problem in order to get an answer to my question?? Thank you very much in advance!