A Cartan-theoretic classification of multiply-transitive $(2,3,5)$-distributions

In his 1910 paper, Élie Cartan gave a tour-de-force solution to the (local) equivalence problem for generic rank 2 distributions on 5-manifolds, i.e. $(2,3,5)$-distributions. From a modern perspective, these structures admit equivalent descriptions as (regular, normal) parabolic geometries modelled on a quotient of $G_2$, but this is not transparent from his article: indeed, the Cartan "connection" of 1910 is not a "Cartan connection" in the modern sense. We revisit the classification of multiply-transitive $(2,3,5)$-distributions from a modern Cartan-geometric perspective, incorporating $G_2$ structure theory throughout, obtaining: (i) the complete (local) classifications in the complex and real settings, phrased "Cartan-theoretically", and (ii) the full curvature and infinitesimal holonomy of all these models. Moreover, we Cartan-theoretically prove exceptionality of the $3:1$ ratio for two 2-spheres rolling on each other without twisting or slipping, yielding a $(2,3,5)$-distribution with symmetry the Lie algebra of the split real form of $G_2$.

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