## A very quick exercise in L'Hopital's rule

2022 October 10

When some ratio has a limit in an “indeterminate form”, in that , L’Hopital’s rule (first discovered by Bernoulli) allows for calculating the limit: under certain conditions. And if at first you don’t succeed, try, try again: when is also in indeterminate form, you can differentiate as many times as needed.

As an aside, if we suppose that and can be approximated as Taylor series near , one can easily find that when the leading terms of and are both zero, representing the case where differentiations are needed. Here represents the differentiation-with-respect-to- operator.

While I have occasionally run into cases where multiple applications of L’Hopital’s rule are needed, the other day1by which I mean 7 years ago… I am a bit slow at updating this blog I was struck by a situation which necessitated a recursive application of L’Hopital’s rule. Consider the Lambert W function which is defined implicitly by the equation that is, is the inverse of the function . We will only be interested in the principal real branch, which is defined for . Note that . Also, per Wikipedia, we have  Plot of the two real branches of the Lambert W function. The principal branch is labeled .

Ok, now the calculation I ran into in my research was: Let and so our goal is to calculate where the limit goes .

Note that . Starting by applying the chain rule to the derivative of , we have From what initially, as I was first calculating it, looked like it would blow up endlessly, instead comes a surprisingly simple and clean result!

Thanks Harry Altman for identifying a mistake in an earlier draft.

1. by which I mean 7 years ago… I am a bit slow at updating this blog↩︎