Simultaneous Partial-Wave Expansion in the Mandelstam Variables: The GroupSU(3)

The elastic scattering amplitude of two spinless particles of equal mass ½ was expanded elsewhere in a double series of eigenfunctions which "displayed" its dependence on all the Mandelstam variables $s$, $t$, $u$ ($s+t+u=1\mathrm{}$). The expansion was then used to investigate the crossing properties of partial-wave amplitudes. We show in this paper that these eigenfunctions are certain basis vectors of the representations ($\sigma , \sigma$) of a suitably defined $\mathrm{SU}\left(3\right)\mathrm{}$. The unequal-mass problem is also discussed.