In mathematics, the Lyusternik–Schnirelmann category (or, Lusternik–Schnirelmann category, LS-category, or simply, category) of a topological space X is the topological invariant defined as the smallest cardinality of an open covering of X by contractible subsets. For example, if X is the circle, this takes the value two.

Recently a different normalisation of the invariant has been adopted, which is one less than the original definition by Lusternik and Schnirelmann. Such a normalisation has been adopted in the definitive monograph by Cornea, Lupton, Oprea, and Tanré (see below).

In general it is not so easy to compute this invariant, which was initially introduced by Lazar Lyusternik and Lev Schnirelmann in connection with variational problems. It has a close connection with algebraic topology, in particular cup-length. In the modern normalisation, the cup-length is a lower bound for LS category.

It was, as originally defined for the case of X a manifold, the lower bound for the number of critical points a Morse function on X could possess (cf. Morse theory).

See also

• Ganea conjecture
• Systolic category

References

• R. H. Fox, On the Lusternik-Schnirelmann category, Annals of Mathematics 42 (1941), 333-370.

• Samuel Eilenberg, Tudor Ganea, On the Lusternik-Schnirelmann category of abstract groups, Annals of Mathematics, 2nd Ser., 65 (1957), no. 3, 517 – 518

• F. Takens, The minimal number of critical points of a function on compact manifolds and the Lusternik-Schnirelmann category, Inventiones Mathematicae 6 (1968), 197-244.

• Tudor Ganea, Some problems on numerical homotopy invariants, Lecture Notes in Math. 249 (Springer, Berlin, 1971), pp. 13 – 22 MR0339147

• Ioan James, On category, in the sense of Lusternik-Schnirelmann, Topology 17 (1978), 331-348.

• Kathryn Hess, A proof of Ganea's conjecture for rational spaces, Topology 30 (1991), no. 2, 205--214. MR1098914

• Norio Iwase, "Ganea's conjecture on Lusternik-Schnirelmann category", in Bulletin of the London Mathematical Society, 30 (1998), no.6, 623 – 634 MR1642747

• Norio Iwase, A_∞-method in Lusternik-Schnirelmann category, Topology 41 (2002), no. 4, 695--723. MR1905835

• Lucile Vandembroucq, Fibrewise suspension and Lusternik-Schnirelmann category, Topology 41 (2002), no. 6, 1239--1258. MR1923222

• Octav Cornea, Gregory Lupton, John Oprea, Daniel Tanré, Lusternik-Schnirelmann category, Mathematical Surveys and Monographs, 103. American Mathematical Society, Providence, RI, 2003 ISBN 0-8218-3404-5

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