Express the following as Gamma functions. Namely, noting the form \(\Gamma(x+1)=\int_{0}^{\infty} t^{x} e^{-t} d t\) and using an appropriate substitution, each expression can be written in terms of a Gamma function.

a. \(\int_{0}^{\infty} x^{2 / 3} e^{-x} d x\).

b. \(\int_{0}^{\infty} x^{5} e^{-x^{2}} d x\).

c. \(\int_{0}^{1}\left[\ln \left(\frac{1}{x}\right)\right]^{n} d x\).