Lefschetz fixed-point theorem

For a formal statement of the theorem, let :f\colon X \rightarrow X\, be a continuous map from a compact triangulable space X to itself. Define the Lefschetz number \Lambda_f of f by :\Lambda_f:=\sum_{k\geq 0}(-1)^k\mathrm{tr}(H_k(f,\Q)), the alternating (finite) sum of the matrix traces of the linear maps induced by f on H_k(X,\Q), the singular homology groups of X with rational coefficients. A simple version of the Lefschetz fixed-point theorem states: if :\Lambda_f \neq 0\, then f has at least one fixed point, i.e., there exists at least one x in X such that f(x) = x. In fact, since the Lefschetz number has been defined at the homology level, the conclusion can be extended to say that any map homotopic to f has a fixed point as well. Note however that the converse is not true in general: \Lambda_f may be zero even if f has fixed points, as is the case for the identity map on odd-dimensional spheres. ==Sketch of a proof==

First, by applying the simplicial approximation theorem, one shows that if f has no fixed points, then (possibly after subdividing X) f is homotopic to a fixed-point-free simplicial map (i.e., it sends each simplex to a different simplex). This means that the diagonal values of the matrices of the linear maps induced on the simplicial chain complex of X must all be zero. Then one notes that, in general, the Lefschetz number can also be computed using the alternating sum of the matrix traces of the aforementioned linear maps (this is true for almost exactly the same reason that the Euler characteristic has a definition in terms of homology groups; see below for the relation to the Euler characteristic). In the particular case of a fixed-point-free simplicial map, all of the diagonal values are zero, and thus the traces are all zero. ==Lefschetz–Hopf theorem==

A stronger form of the theorem, also known as the Lefschetz–Hopf theorem, states that, if f has only finitely many fixed points, then :\sum_{x \in \mathrm{Fix}(f)} \mathrm{ind}(f,x) = \Lambda_f, where \mathrm{Fix}(f) is the set of fixed points of f, and \mathrm{ind}(f,x) denotes the index of the fixed point x. From this theorem one may deduce the Poincaré–Hopf theorem for vector fields as follows. Any vector field on a compact manifold induces a flow \varphi(x,t) in a natural way, and for every t the map \varphi(x,t) is homotopic to the identity (thus having the same Lefschetz number); moreover, for sufficiently small t the fixed points of the flow and the zeroes of the vector field have the same indices. ==Relation to the Euler characteristic==

The Lefschetz number of the identity map on a finite CW complex can be easily computed by realizing that each f_\ast can be thought of as an identity matrix, and so each trace term is simply the dimension of the appropriate homology group. Thus the Lefschetz number of the identity map is equal to the alternating sum of the Betti numbers of the space, which in turn is equal to the Euler characteristic \chi(X). Thus we have :\Lambda_{\mathrm{id}} = \chi(X).\ ==Relation to the Brouwer fixed-point theorem==

The Lefschetz fixed-point theorem generalizes the Brouwer fixed-point theorem, which states that every continuous map from the n-dimensional closed unit disk D^n to D^n must have at least one fixed point. This can be seen as follows: D^n is compact and triangulable, all its homology groups except H_0 are zero, and every continuous map f\colon D^n \to D^n induces the identity map f_* \colon H_0(D^n, \Q) \to H_0(D^n, \Q), whose trace is one; all this together implies that \Lambda_f is non-zero for any continuous map f\colon D^n \to D^n. ==Historical context==

Lefschetz presented his fixed-point theorem in . Lefschetz's focus was not on fixed points of maps, but rather on what are now called coincidence points of maps. Given two maps f and g from an orientable manifold X to an orientable manifold Y of the same dimension, the Lefschetz coincidence number of f and g is defined as :\Lambda_{f,g} = \sum (-1)^k \mathrm{tr}( D_X \circ g^* \circ D_Y^{-1} \circ f_*), where f_* is as above, g^* is the homomorphism induced by g on the cohomology groups with rational coefficients, and D_X and D_Y are the Poincaré duality isomorphisms for X and Y, respectively. Lefschetz proved that if the coincidence number is nonzero, then f and g have a coincidence point. He noted in his paper that letting X= Y and letting g be the identity map gives a simpler result, which is now known as the fixed-point theorem. ==Frobenius==

Let X be a variety defined over the finite field k with q elements and let \bar X be the base change of X to the algebraic closure of k. The Frobenius endomorphism of \bar X (often the geometric Frobenius, or just the Frobenius), denoted by F_q, maps a point with coordinates x_1,\ldots,x_n to the point with coordinates x_1^q,\ldots,x_n^q. Thus the fixed points of F_q are exactly the points of X with coordinates in k; the set of such points is denoted by X(k). The Lefschetz trace formula holds in this context, and reads: :\#X(k)=\sum_i (-1)^i \mathrm{tr}(F_q^*| H^i_c(\bar{X},\Q_{\ell})). This formula involves the trace of the Frobenius on the étale cohomology, with compact supports, of \bar X with values in the field of \ell-adic numbers, where \ell is a prime coprime to q. If X is smooth and equidimensional, this formula can be rewritten in terms of the arithmetic Frobenius \Phi_q, which acts as the inverse of F_q on cohomology: :\#X(k)=q^{\dim X}\sum_i (-1)^i \mathrm{tr}((\Phi_q^{-1})^*| H^i(\bar X,\Q_\ell)). This formula involves usual cohomology, rather than cohomology with compact supports. The Lefschetz trace formula can also be generalized to algebraic stacks over finite fields. ==See also==