Categories for the Working Mathematician
《Categories for the Working Mathematician》 · 第 137 页 · Chapter VI, Section 1
Any endofunctor \(T:\mathcal X\to\mathcal X\) has composites \(T^2=T\circ T:\mathcal X\to\mathcal X\) and \(T^3=T^2T:\mathcal X\to\mathcal X\).