Lusternik-Schnirelmann category and systolic category of low-dimensional manifolds

We show that the geometry of a Riemannian manifold (M, G) is sensitive to the apparently purely homotopy-theoretic invariant of M known as the Lusternik-Schnirelmann category, denoted cat _LS(M). Here we introduce a Riemannian analogue of cat _LS (M), called the systolic category of M. It is denoted cat _sys (M) and defined in terms of the existence of systolic inequalities satisfied by every metric G, as initiated by C. Loewner and later developed by M. Gromov. We compare the two categories. In all our examples, the inequality cat _sys M ≤ cat _LS M is satisfied, which typically turns out to be an equality, e.g., in dimension 3. We show that a number of existing systolic inequalities can be reinterpreted as special cases of such equality and that both categories are sensitive to Massey products. The comparison with the value of cat _LS (M) leads us to prove or conjecture new systolic inequalities on M.