problem with calculation limit

Hi, I have recently used matlab to calculate the limit of the following function when E goes to minus infinity:

-E*(log(1/(exp(3*E) + 1))/(exp(3*E) + 1) - log(1/(exp(3*E + 3*a) + 1))/(exp(3*E + 3*a) + 1))

But Matlab returns:

limit(-E*(log(1/(exp(3*E) + 1))/(exp(3*E) + 1) - log(1/(exp(3*E + 3*a) + 1))/(exp(3*E + 3*a) + 1)), E == -Inf)

Does anybody knows what is going on? Am I doing something wrong?

Answer 1

[Roger Stafford]

I would think the limit is zero by L'Hospital's rule, but it is possible that matlab's 'limit' function either doesn't know how to determine that or else it objects to the presence of the unknown variable 'a'. Try giving 'a' a specific value and see how it performs.