alright, consider the gamma function:
Γ(z)=∫(t^(z-1)e^(-t)dt, 0, infinity)
which has the property Γ(z+1)=zΓ(z) (too long to prove, look it up),
in turn implying that Γ(z+1)=z!
so 0!=Γ(0+1)=Γ(1)
Γ(1)=∫(t^(1-1)e^(-t)dt, 0, infinity)
=∫(e^(-t)dt, 0, infinity)
= -e^(-t), 0, infinity...