Because the double factorial is not the same as the factorial applied twice. https://en.wikipedia.org/wiki/Double_factorial https://en.wikipedia.org/wiki/Doubl

Because the double factorial is not the same as the factorial applied twice. https://en.wikipedia.org/wiki/Double_factorial https://en.wikipedia.org/wiki/Double_factorial sin⁻¹(x) as a notation for arcsin(x) is another notation annoyance, and far more widely used than n!!. sin⁻¹(x) is not 1/sin(x) even though sin²(x) = sin(x) sin(x). While for non-trig functions, f²(x) = f(f(x))