2⁢ω3=ⅈ⁢∫e2e1ⅆ⁡u(e1-u)⁢(u-e2)⁢(u-e3)=ⅈ⁢∫-∞e3ⅆ⁡u(e1-u)⁢(e2-u)⁢(e3-u).