The subspace is spanned by E, P →, M and L ij. We already know how the subspace of the irrep transforms under all operators but the angular momentum. Note that the rotation subgroup is Spin(3). We have to look at its double cover, because we are considering projective representations. This is called the little group, a name given by Eugene Wigner. His method of induced representations specifies that the irrep is given by the direct sum of all the fibers in a vector bundle over the mE = mE 0 + P /2 hypersurface, whose fibers are a unitary irrep of Spin(3).