Specific question

Dear members  of Bug-gne -- GNUPedia technical discussions list,

I have a question related to fitting the equation to experimental   
data. The fitting equation looks like y=a*exp(-(b*ln(c/x)/e)**(d)).
Only three parameters have to be fitted, these are a, e, d.   
Experimental data are y and x.The coefficients b and c are known.  
After writing mentioned above function into script file, the program   
says that 'ln' is undefined function. I used logarithmic and normal   
scale for x axis, but the message was the same. Why it is so?

How should I describe such a function? I could not find the answer in
http://t16web.lanl.gov/Kawano/gnuplot/index-e.html

Please help me to solve my problem.

Diana