Lefschetz' fixed-point theorem, or the Lefschetz–Hopf theorem, is a theorem that makes it possible to express the number of fixed points of a continuous mapping in terms of its Lefschetz number. Thus, if a continuous mapping of a finite CW-complex (cf. also Cellular space) has no fixed points, then its Lefschetz number is equal to zero. A special case of this assertion is Brouwer's fixed-point theorem (cf. Brouwer theorem).
|[a1]||M.J. Greenberg, J.R. Harper, "Algebraic topology, a first course" , Benjamin/Cummings (1981) MR643101 Zbl 0498.55001|
Lefschetz' hyperplane-section theorem, or the weak Lefschetz theorem: Let be an algebraic subvariety (cf. Algebraic variety) of complex dimension in the complex projective space , let be a hyperplane passing through all singular points of (if any) and let be a hyperplane section of ; then the relative homology groups (cf. Homology group) vanish for . This implies that the natural homomorphism
is an isomorphism for and is surjective for (see ).
Using universal coefficient formulas (cf. Künneth formula) one obtains corresponding assertions for arbitrary cohomology groups. In every case, for cohomology with coefficients in the field of rational numbers the dual assertions hold: The homomorphism of cohomology spaces
induced by the imbedding is an isomorphism for and is injective for (see ).
An analogous assertion is true for homotopy groups: for . In particular, the canonical homomorphism is an isomorphism for and is surjective for (the Lefschetz theorem on the fundamental group). There is a generalization of this theorem to the case of an arbitrary algebraically closed field (see ), and also to the case when is a normal complete intersection of (see ).
The hard Lefschetz theorem is a theorem about the existence of a Lefschetz decomposition of the cohomology of a complex Kähler manifold into primitive components.
Let be a compact Kähler manifold of dimension with Kähler form , let
be the cohomology class of type corresponding to under the de Rham isomorphism (cf. de Rham cohomology; if is a projective algebraic variety over with the natural Hodge metric, then is the cohomology class dual to the homology class of a hyperplane section) and let
be the linear operator defined by multiplication by , that is,
One has the isomorphism (see )
for any . The kernel of the operator
is denoted by and is called the primitive part of the -cohomology of the variety . The elements of are called primitive cohomology classes, and the cycles corresponding to them are called primitive cycles. The hard Lefschetz theorem establishes the following decomposition of the cohomology into the direct sum of primitives (called the Lefschetz decomposition):
for all . The mappings
are imbeddings. The Lefschetz decomposition commutes with the Hodge decomposition (cf. Hodge conjecture)
(see ). In particular, the primitive part of is defined and
The hard Lefschetz theorem and the Lefschetz decomposition have analogues in abstract algebraic geometry for -adic and crystalline cohomology (see , ).
The Lefschetz theorem on cohomology of type is a theorem about the correspondence between the two-dimensional algebraic cohomology classes of a complex algebraic variety and the cohomology classes of type .
Let be a non-singular projective algebraic variety over the field . An element is said to be algebraic if the cohomology class dual to it (in the sense of Poincaré) is determined by a certain divisor. The Lefschetz theorem on cohomology of type asserts that a class is algebraic if and only if
where is the Hodge component of type of the two-dimensional complex cohomology space , and the mapping is induced by the natural imbedding (see , and also , ). For algebraic cohomology classes in dimensions greater than 2, see Hodge conjecture.
For an arbitrary complex-analytic manifold there is an analogous characterization of elements of the group that are Chern classes of complex line bundles over (see ).
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|||S. Lefschetz, "On certain numerical invariants of algebraic varieties with applications to Abelian varieties" Trans. Amer. Math. Soc. , 22 (1921) pp. 327–482 MR1501180 MR1501178|
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|||R. Hartshorne, "Ample subvarieties of algebraic varieties" , Springer (1970) MR0282977 Zbl 0208.48901|
|||D. Mumford, "Abelian varieties" , Oxford Univ. Press (1974) MR2514037 MR1083353 MR0352106 MR0441983 MR0282985 MR0248146 MR0219542 MR0219541 MR0206003 MR0204427 Zbl 0326.14012|
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|||R.O. Wells jr., "Differential analysis on complex manifolds" , Springer (1980) MR0608414 Zbl 0435.32004|
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|||P. Deligne, "La conjecture de Weil" Publ. Math. IHES , 43 (1974) pp. 273–307 MR0340258 Zbl 0456.14014 Zbl 0314.14007 Zbl 0287.14001 Zbl 0219.14022|
For a modern treatment of the classical Lefschetz hyperplane-section theorems see [a1].
|[a1]||K. Lamotke, "The topology of complex projective varieties after S. Lefschetz" Topology , 20 (1981) pp. 15–51 MR0592569 Zbl 0445.14010|
|[a2]||P. Deligne, "La conjecture de Weil II" Publ. Math. IHES , 52 (1980) pp. 137–252 MR0601520 Zbl 0456.14014|
|[a3]||M.J. Greenberg, J.R. Harper, "Algebraic topology, a first course" , Benjamin/Cummings (1981) MR643101 Zbl 0498.55001|
|[a4]||J.S. Milne, "Etale cohomology" , Princeton Univ. Press (1980) MR0559531 Zbl 0433.14012|
|[a5]||M. Goresky, "Stratified Morse theory" , Springer (1988) MR0932724 Zbl 0639.14012|
|[a6]||A. Beilinson, J. Bernstein, P. Deligne, "Faisceaux pervers" Astérisque , 100 (1982) MR0751966 Zbl 0536.14011|
Lefschetz theorem. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Lefschetz_theorem&oldid=23883