= Orthogonal-invariant scattering channels
{title2=$s_0=NS_1+S_2+S_3,\quad s_\pm=S_2\pm S_3$}
The two-vector <tensor product> splits into a trace singlet, symmetric traceless tensors and antisymmetric tensors. For an <O(N)-invariant S-matrix>, the corresponding <eigenvalues> are $s_0=NS_1+S_2+S_3$, $s_+=S_2+S_3$ and $s_-=S_2-S_3$. For $N\ge2$, analytic <unitarity> requires $s_a(\theta)s_a(-\theta)=1$. With <Hermitian analyticity of a two-particle S-matrix>, each channel has unit modulus on the real <rapidity> axis.
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