# Thread: Differentiating sum of log

1. ## Differentiating sum of log

Hi everyone,

This is my first post, so please bear with me if I am posting in the wrong forum.

I study economics and I am reading an article. I do not understand how the author gets the following conclusions:

He takes log of this function:

And ends up with:

I thought it would be instead of the log[T!/(T-t)].

Secondly I want do differentiate logc_t^t, but I am unsure how to deal with the log sum expressions. How do you differentiate ??

Wolfram alpha uses gamma functions which I honestly do not understand. The article I am reading finds the following derivative:

I hope someone can clearify this for me. Thank you very much!

Jacob

2. ## Re: Differentiating sum of log

Hey Katmarn.

Try expressing that term in terms of factorials and see if it is equivalent.

3. ## Re: Differentiating sum of log

Thank you Chiro.

I had tried it before without luck, but as I did it again it suddently made sense

However, I still do not know how you take the derivative of a factorial term. Could you please show me how you do it on:

When I express it in terms of factorials I get: log((1+b^(1/n)*T)!/[1+b^(1/n)*(T-t-1)]!), and I have no idea how to take the derivative of that...

Thank you!

4. ## Re: Differentiating sum of log

If you are differentiating a factorial, take a look at the Euler Gamma function and use the fundamental theorem of calculus:

Gamma function - Wikipedia, the free encyclopedia

5. ## Re: Differentiating sum of log

Thank you for the answer, but I must admit I am still lost as I do not fully understand the gamma function.

Could you, or someone else, please explain the steps the author must use to get his result?

He takes the derivative of: wrt t.

And he ends up with:

I have tried to express the sum with factorials, and got:

I do not know if this is even possible as 0<beta<1. However, if it is correct i can split the log up and just focus on the bottom part when i want to take the derivative wrt. t.

And here I am stuck...

All help is appreciated.

6. ## Re: Differentiating sum of log

The Gamma function is the factorial function, but is extended for all real values instead of only being valid for integer values. It is not defined at 0 or the negative integers, but it is defined everywhere else.

The connection is that n! = Gamma(n+1).

7. ## Re: Differentiating sum of log

Ok. Do you have the time to explain how the author ends up with the result that he does?

8. ## Re: Differentiating sum of log

I'll take a look at this later on so if I haven't responded tomorrow, just bump the thread.

Chiro

10. ## Re: Differentiating sum of log

Try substituting for s values t - 1, t - 2, ..., t - t and use the fact that d/dx(u + v) = du/dx + dv/dx.

Then collect the derivatives of all the logarithms and see if you get the same answer.