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> There they commute iff... $[G,H]$ is a constant multiple of the identity. I understand $\mathbb{P}\mathcal{H}$ to contain the equivalence classes of vectors related by a multiplicative constant. But then, isn't calling $[G,H]$ a "constant multiple of the identity" the same as saying $[G,H]=0$? It seems to me that on the projective space $\mathbb{P}\mathcal{H}$, it is also the case that $[e^{ibG}, e^{iaH}]=0$ iff $[G,H]=0$. What am I missing? [P.S.: Thanks for the Roels and Weinstein reference, that is very helpful.]
