Dalitz boundary from pair-rest-frame kinematics ($s_{23}^{\pm }=m_2^2+m_3^2+2(E_2^{\ast }{E}_3^{\ast }\pm p_2^{\ast }{p}_3^{\ast })$)

At fixed $s=s_{13}$, use $E_2^{\ast }=(M^2-m_2^2-s)/(2\sqrt{s})$ and $E_3^{\ast }=(s+m_3^2-m_1^2)/(2\sqrt{s})$ in the 13-pair rest frame. The endpoint values of its relative cosine give the displayed Dalitz plot boundary. The momenta are fixed by the Källén function; $(m_1+m_3{)}^2\le s\le (M-m_2{)}^2$. Integrating $s_{23}^{+}-s_{23}^{-}$ gives the invariant-coordinate area, whose phase-space normalization still contains $M^{-2}$.