Remarks on the $P_c$ structures and triangle singularities

It was proposed that the narrow $P_c(4450)$ structure observed by the LHCb Collaboration in the reaction ${\mathrm{\Lambda }}_b\to J\mathrm{/}\psi pK$ might be due to a triangle singularity around the ${\chi }_{c1}$–proton threshold at 4.45~GeV. We discuss the occurrence of a similar triangle singularity in the $J\mathrm{/}\psi p$ invariant mass distribution for the decay ${\mathrm{\Lambda }}_b\to J\mathrm{/}\psi p\pi$, which could explain the bump around 4.45~GeV in the data. More precise measurements of this process would provide valuable information towards an understanding of the $P_c$ structures.