Spectrum of spaces
A spectrum of spaces is defined as a sequence of topological (as a rule cellular) spaces together with mappings , where denotes suspension. Spectra of spaces form a category; a morphism of a spectrum into a spectrum is, roughly speaking, the "cofinal part" of some function given by a family of mappings with ( is to as is to ). The concepts of homotopic morphisms and homotopy equivalent spectra of spaces may be introduced, and one may construct the homotopy category of spectra . Postnikov systems (cf. Postnikov system) of spectra of spaces have also been introduced.
The suspension over a spectrum of spaces is defined as the spectrum of spaces . Let . Then and are homotopically mutually inverse functors, so that in the category of spectra of spaces (in contrast to the category of spaces), the suspension functor is invertible, and this makes it convenient to work in the former. In general, in the category of spectra of spaces, all arguments connected with stability (e.g. the construction of the Adams spectral sequence) assume a natural form.
Examples of spectra of spaces.
1) For any space one can define a spectrum of spaces , where when and when , and is the natural identification . So, for one obtains the spectrum of spheres .
2) The spectrum of the Eilenberg–MacLane spaces (or ), where is an Abelian group. The homotopy equivalence , where is an Eilenberg–MacLane space and is the loop space over , gives an adjoint mapping , so that one obtains the spectrum of spaces . This spectrum of spaces represents the ordinary cohomology theory with coefficients in .
3) Let be a space such that for some . For , , , let . There arises a sequence of the form . As in example 2), the homotopy equivalence gives a mapping , so that one obtains a spectrum of spaces. E.g., for the classifying space , where the are unitary groups, (Bott's periodicity theorem), and one obtains the spectrum of spaces representing complex -theory. An analogous result holds for real -theory ().
Given two spectra of spaces and , one can define their reduced product (an analogue of the usual reduced product of spaces). A multiplication on is defined as a morphism which is associative (in the appropriate sense). A spectrum of spaces equipped with a multiplication is called a ring spectrum, or multiplicative spectrum, and the cohomology theory represented by it is multiplicative. Attempts to overcome the difficulties connected with the "poor associativity" of the above multiplication have led to a revision of the foundations of the theory of spectra of spaces. Namely, the concept of a coordinate-free spectrum of spaces is introduced as a family of spaces (and corresponding mappings) indexed by linear subspaces of . The category of coordinate-free spectra of spaces is isomorphic to the category of ordinary spectra of spaces, but the pairing in it is easier to control, and so it plays an important role in the examination of subtle geometric questions connected with the higher structures of spectra of spaces, with orientations in cohomology theory, and elsewhere.
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A mapping of spectra is defined by individual continuous mappings (or for a mapping of spectra of degree ). Applying the mapping-cone construction and mapping cylinder construction to these individual mappings defines the mapping cone of a mapping of spectra and the mapping cylinder of a mapping of spectra.
A spectrum is an -spectrum if the mappings (adjoint to ) are weak homotopy equivalences.
Often, a CW-spectrum is defined as a sequence of CW-complexes such that is (or, is homotopic to) a subcomplex of .
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Spectrum of spaces. Yu.B. Rudyak (originator), Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Spectrum_of_spaces&oldid=12104