Let ,
with
. Set
. It is easy to see that
F(s) is defined for s > 0. Consider the new variable r = st.
Then we have
,
which implies
,
where the Gamma function is defined by
The domain of the Gamma function is x > 1. Using integration by part, one can easily prove the fundamental formula
,
which implies (knowing that )
Consequently, we have
If you are interested to learn more about the Gamma function, click HERE.