 Solution

The connected $(1+2)$-dimensional Poincare group isThe semidirect product records that Lorentz transformations act nontrivially on translations, which is what organizes momentum orbits. For a representative momentum $p$, its little group is the stabilizer $G_p=\{\Lambda \in S{O}^{+}(1,2):\Lambda p=p\}$. Wigner's classification constructs irreducible particle representations by inducing a unitary irreducible representation of $G_p$ along the Lorentz orbit of $p$.