A formalism for the study of $K^{+}\pi \mathrm{\Sigma }$ photoproduction in the ${\mathrm{\Lambda }}^{\ast }(1405)$ region

We present a formalism for the low-energy analysis of the $\gamma p\rightarrow K^{+}\pi\Sigma$photoproductionreactioninthe$\Lambda^{\ast}(1405)$ resonance region. In particular, the constraints arising from unitarity, gauge invariance and chiral perturbation theory are discussed in some detail.