You're making a mistake when changing to .
should give you:
The expression for the Gamma function is given as:
for Re(z)>0, and we change the variable into
It then says that by this, we get:
Which I am not sure how they got.
Here is what I tried:
Then I assume and
Substituting variables, I get:
Which is not the same as what I'm suppose to get.
If someone could show how it should be done, that would be greatly appreciated.