functions can either be even or odd. let ϕ(x)\phi(x)ϕ(x) be a function even: ϕ(−x)=ϕ(x)\phi(-x) = \phi(x)ϕ(−x)=ϕ(x) odd: ϕ(−x)=−ϕ(x)\phi(-x) = -\phi(x)ϕ(−x)=−ϕ(x)Flow of Probability