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Lefschetz formula

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A formula that expresses the number of fixed points of an endomorphism of a topological space in terms of the traces of the corresponding endomorphisms in the cohomology groups.

This formula was first established by S. Lefschetz for finite-dimensional orientable topological manifolds [1] and for finite cell complexes (see [2], [3]). These papers of Lefschetz were preceded by a paper of L.E.J. Brouwer (1911) on the fixed point of a continuous mapping of an - dimensional sphere into itself. A new version of the proof of the Lefschetz formula for finite cell complexes was given by H. Hopf (see [9]).

Let X be a connected orientable n - dimensional compact topological manifold or an n - dimensional finite cell complex, let f : X \rightarrow X be a continuous mapping and let \Lambda ( f , X ) be the Lefschetz number of f . Assume that all fixed points of the mapping f : X \rightarrow X are isolated. For each fixed point x \in X , let i ( x) be its Kronecker index (the local degree (cf. Degree of a mapping) of f in a neighbourhood of x ). Then the Lefschetz formula for X and f has the form

\tag{1 } \sum _ {f ( x) = x } i ( x) = \Lambda ( f , X ) .

There is, [8], a generalization of the Lefschetz formula to the case of arbitrary continuous mappings of compact Euclidean neighbourhood retracts.

Let X be a differentiable compact orientable manifold and let f : X \rightarrow X be a differentiable mapping. A fixed point x \in X for f is said to be non-singular if it is isolated and if \mathop{\rm det} ( df _ {x} - E ) \neq 0 , where df _ {x} : T _ {x} ( X) \rightarrow T _ {x} ( X) is the differential of f at x and E is the identity transformation. For a non-singular point x \in X its index i ( x) coincides with the number \mathop{\rm sgn} \mathop{\rm det} ( df _ {x} - E ) . In this case the Lefschetz formula (1) shows that the Lefschetz number \Lambda ( f , X ) is equal to the difference between the number of fixed points with index + 1 and the number of fixed points with index - 1 ; in particular, it does not exceed the total number of fixed points. In this case the left-hand side of (1) can be determined in the same way as the intersection index \Gamma _ {f} \Delta on X \times X , where \Gamma _ {f} is the graph of f and \Delta \subset X \times X is the diagonal (cf. Intersection index (in algebraic geometry)).

A consequence of the Lefschetz formula is the Hopf formula, which asserts that the Euler characteristic \chi ( X) is equal to the sum of the indices of the zeros of a global C ^ \infty - vector field v on X ( it is assumed that all zeros of v are isolated) (see [5]).

There is a version of the Lefschetz formula for compact complex manifolds and the Dolbeault cohomology (see [5]). Let X be a compact complex manifold of dimension m and let f : X \rightarrow X a be holomorphic mapping with non-singular fixed points. Let H ^ {p,q} ( X) be the Dolbeault cohomology of X of type ( p , q ) ( cf. Differential form) and let f ^ { * } : H ^ {p,q} ( X) \rightarrow H ^ {p,q} ( X) be the endomorphism induced by f . The number

\Lambda ( f , {\mathcal O} _ {X} ) = \sum _ { q= } 0 ^ { m } (- 1) ^ {q} \mathop{\rm Tr} ( f ^ { * } ; H ^ {0,q} ( X) )

is called the holomorphic Lefschetz number. One then has the following holomorphic Lefschetz formula:

\Lambda ( f , {\mathcal O} _ {X} ) = \sum _ {f ( x) = x } \frac{1}{ \mathop{\rm det} ( E - df _ {x} ) } ,

where df _ {x} is the holomorphic differential of f at x .

In abstract algebraic geometry the Lefschetz formula has served as a starting point in the search for Weil cohomology in connection with Weil's conjectures about zeta-functions of algebraic varieties defined over finite fields (cf. Zeta-function). An analogue of the Lefschetz formula in abstract algebraic geometry has been established for l - adic cohomology with compact support and with coefficients in constructible \mathbf Q _ {l} - sheaves, where \mathbf Q _ {l} is the field of l - adic numbers and where l is a prime number distinct from the characteristic of the field k . This formula is often called the trace formula.

Let X be an algebraic variety (or scheme) over a finite field k , let F : X \rightarrow X be a Frobenius morphism (cf. e.g. Frobenius automorphism), {\mathcal F} a sheaf on X , and let H _ {c} ^ {i} ( X , {\mathcal F} ) be cohomology with compact support of the variety (scheme) X with coefficients in {\mathcal F} . Then the morphism F determines a cohomology endomorphism

F ^ { * } : H _ {c} ^ {i} ( X , {\mathcal F} ) \rightarrow H _ {c} ^ {i} ( X ,\ {\mathcal F} ) .

If k _ {n} \supset k is an extension of k of degree n and if X _ {n} = X \otimes k _ {n} , {\mathcal F} _ {n} = {\mathcal F} \otimes k _ {n} are the variety (scheme) and sheaf obtained from X and {\mathcal F} by extending the field of scalars, then the corresponding Frobenius morphism F _ {n} : X _ {n} \rightarrow X _ {n} coincides with the n - th power F ^ { n } of F .

Now let X be a separable scheme of finite type over the finite field k of q elements, let {\mathcal F} be a constructible \mathbf Q _ {l} - sheaf on X , l a prime number distinct from the characteristic of k , and X ^ {F ^ {n} } the set of fixed geometric points of the morphism F ^ { n } or, equivalently, the set X ( k _ {n} ) of geometric points of the scheme X with values in the field k _ {n} . Then for any integer n \geq 1 the following Lefschetz formula (or trace formula) holds (see [6], [7]):

\tag{2 } \sum _ {x \in X ^ {F ^ {n} } } \mathop{\rm Tr} ( F ^ { n* } , {\mathcal F} _ {x} ) = \ \sum _ { i } (- 1) ^ {i} \mathop{\rm Tr} ( F ^ { * n } , H _ {c} ^ {i} ( X ,\ {\mathcal F} )) ,

where {\mathcal F} _ {x} is the stalk of {\mathcal F} over x . In the case of the constant sheaf {\mathcal F} = \mathbf Q _ {l} one has \mathop{\rm Tr} ( F ^ { n* } , \mathbf Q _ {l} ) = 1 and the left-hand side of (2) is none other than the number of geometric points of X with values in k _ {n} . In particular, for n= 1 this is simply the number of points of X with values in the ground field k . If X is proper over k ( for example, if X is a complete algebraic variety over k ), then H _ {c} ^ {i} ( X , {\mathcal F} ) = H ^ {i} ( X , {\mathcal F} ) and the right-hand side of (2) is an alternating sum of the traces of the Frobenius endomorphism in the ordinary cohomology of X .

There are (see [7]) generalizations of formula (2).

Comments

For the Lefschetz formula in abstract algebraic geometry and its generalizations by A. Grothendieck see also [a1].

References

[1] S. Lefschetz, "Intersections and transformations of complexes and manifolds" Trans. Amer. Soc. , 28 (1926) pp. 1–49 MR1501331 Zbl 52.0572.02
[2] S. Lefschetz, "The residual set of a complex manifold and related questions" Proc. Nat. Acad. Sci. USA , 13 (1927) pp. 614–622 Zbl 53.0553.01
[3] S. Lefschetz, "On the fixed point formula" Ann. of Math. (2) , 38 (1937) pp. 819–822 MR1503373 Zbl 0018.17703 Zbl 63.0563.02
[4] S.L. Kleiman, "Algebraic cycles and the Weil conjectures" A. Grothendieck (ed.) J. Giraud (ed.) et al. (ed.) , Dix exposés sur la cohomologie des schémas , North-Holland & Masson (1968) pp. 359–386 MR0292838 Zbl 0198.25902
[5] P.A. Griffiths, J.E. Harris, "Principles of algebraic geometry" , Wiley (Interscience) (1978) MR0507725 Zbl 0408.14001
[6] P. Deligne, "Cohomologie étale (SGA 4 1/2)" , Lect. notes in math. , 569 , Springer (1977)
[7] A. Grothendieck, I. Bucur, C. Honzel, L. Illusie, J.-P. Jouanolou, J.-P. Serre, "Cohomologie \ell-adique et fonctions L. SGA 5" , Lect. notes in math. , 589 , Springer (1977) MR491704
[8] A. Dold, "Lectures on algebraic topology" , Springer (1980) MR0606196 Zbl 0434.55001
[9] H. Seifert, W. Threlfall, "A textbook of topology" , Acad. Press (1980) (Translated from German) MR0575168 Zbl 0469.55001
[a1] E. Feitag, R. Kiehl, "Etale cohomology and the Weil conjecture" , Springer (1988) MR926276
How to Cite This Entry:
Lefschetz formula. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Lefschetz_formula&oldid=53480
This article was adapted from an original article by V.A. Iskovskikh (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article