Gamma(1)=1

Theorem: The following formula holds: $$\Gamma(1)=1,$$ where $\Gamma$ denotes the gamma function.

Proof: Compute $$\begin{array}{ll} \Gamma(1) &= \displaystyle\int_0^{\infty} \xi^{1-1} e^{-\xi} d\xi \\ &= \displaystyle\int_0^{\infty} e^{-\xi} d\xi \\ &= \left[ -e^{-\xi} \right]_{0}^{\infty} \\ &= 1. \end{array}$$ as was to be shown. █