The systole (or systolic category) is a numerical invariant of a closed manifoldM, introduced by Mikhail Katz and Yuli Rudyak in 2006, by analogy with the Lusternik–Schnirelmann category. The invariant is defined in terms of the systoles of M and its covers, as the largest number of systoles in a product yielding a curvature-free lower bound for the total volume of M. The invariant is intimately related to the Lusternik-Schnirelmann category. Thus, in dimensions 2 and 3, the two invariants coincide. In dimension 4, the systolic category is known to be a lower bound for the Lusternik–Schnirelmann category.
Bibliography
Dranishnikov, Alexander N.; Rudyak, Yuli B. (2009). "Stable systolic category of manifolds and the cup-length". Journal of Fixed Point Theory and Applications. 6 (1): 165–177. arXiv:0812.4637. doi:10.1007/s11784-009-0118-5.