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The calculation of the homotopy type of the space of continuous mappings is a fundamental problem of homotopy theory. The set of path components, corresponds to the homotopy classes of such mappings. There are relatively few cases for which this information is explicitly known (as of 1998). A major impact of the work [a1] of J. Lannes on unstable modules and the T-functor has been to expand this knowledge to include many cases in which the sources and targets are classifying spaces of finite and compact Lie groups (cf. also Lie group).

The work of N. Steenrod and others assigns in a natural way to each topological space and each prime number an algebraic model, consisting of a graded algebra over and an algebra of natural operations, called the Steenrod algebra. Each induces an element that commutes with the action of . is a connected graded Hopf algebra acting on the graded algebra .

The hypothesis that is the cohomology of a space imposes an additional "unstable" condition. This is most simply stated if : is generated as an (non-commutative) algebra by the Steenrod operations , with relations forced by its actions of the cohomology of all topological spaces. For example, and , the modulo- Bockstein operator. The unstable condition is then that for and . The algebraic category of unstable algebras over is thus an approximation to the homotopy category of topological spaces. The larger category of unstable modules over has also proved useful.

For , the structure of and unstable actions are similar, but slightly more involved. However, in all cases, the set of relations in the Steenrod algebra and the unstable condition are derivable from the known action of on the cohomology of products of copies of . In the following, explicit references to the coefficients are omitted.

The relationship of to its model is of particular interest. The equivalence

raises the hope that in very favourable cases the mapping

might be an isomorphism. That suggests that in the category , should be approximated by the left adjoint functor to tensoring on the right by . This motivated J. Lannes to define the functor as follows: If is a finite-dimensional -vector space, then the -functor is the left adjoint in of the functor . In the topological case, there is a natural mapping

For general , the adjoint to accounts for only part of the starting page of a Bousfield–Kan unstable Adams spectral sequence for . Lannes provides the basic connection to topology by blending the algebraic properties of and with the Bousfield–Kan spectral sequence: For many interesting spaces ,

In particular,

For , one has the path component of functions homotopic to . The analogous -construct is as follows: Each induces a -module structure on and

The most striking features of are summarized below (see also [a1]). To some extent, these were presaged by work of G. Carlsson and H.T. Miller, who established that the are injectives in .

a) is exact.

b) respects tensor products, i.e .

c) commutes with the th power operations in a suitable sense.

d) maps to .

In principle, can be calculated by using the exactness property and a resolution of by free unstable -modules. In practice, other methods are often more effective; for example,

1) If is finite, then .

2) If , then

for and finite-dimensional -vector spaces.

3) If in is an inclusion, then is the smallest sub-Hopf algebra of that contains .

4) If is a finite -complex with fixed point set and is the modulo cohomology of the Borel construction, then in .

5) If is a compact Lie group, then

These examples each have powerful topological consequences. For example, the first and fourth lead to new proofs of the Sullivan conjecture, originally proved by Miller and Carlsson. The last leads to a new view of the homotopy theory of classifying spaces. Most of the above is referenced in [a2].


[a1] J. Lannes, "Sur les espaces fonctionnels dont la source est le classifiant d'un -groupe abélien élémentaire" Inst. Hautes Etudes Sci. Publ. Math. , 75 (1992) pp. 135–244 (Appendix by M. Zisman)
[a2] L. Schwartz, "Unstable modules over the Steenrod algebra and Sullivan's fixed point set conjecture" , Univ. Chicago Press (1994)
How to Cite This Entry:
Lannes-T-functor. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by Clarence W. Wilkerson, Jr. (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article