Past exam of the mathematics course of the University of Cambridge / 2005 / iii / Paper 22 / 1 / c

Let ${\mathrm{Map}}_{\mathrm{sp}}(D,E)$ be the space of point-set maps of topological spectra. Its underlying set is the first set in the comparison. A member is a compatible collection $D_j\to E_j$, rather than independently chosen maps at the levels.

Because the source is cellular and the target is an Omega-spectrum, taking spectrum homotopy classes gives all maps in the stable homotopy category:Thus the first set maps onto the second. Two point-set maps have the same stable class precisely when they are spectrum-homotopic, using the cellular cylinder and fibrant target. The homotopy must commute with the structure maps throughout; unrelated levelwise homotopies are not the definition.

A stable map $u:D\to E$ induces a degree-preserving additive natural transformation of represented cohomology theories by postcomposition with ${\Sigma }^{q}u$ in degree $q$. This transformation commutes with the suspension isomorphisms. The resulting mapis onto, but need not be one-to-one. Its kernel consists of hyperphantom maps of spectra: maps $u$ for which $uv=0$ for every stable mapfor every based space $X$ and every $j\ge 0$. Equivalently, they induce zero in all degrees of represented cohomology theory on all based spaces. ThereforeThis condition tests arbitrary suspension spectra with negative shifts. Merely testing finite spectra defines a phantom map of spectra, which is a weaker condition. In particular the homology conclusion in Question 5 should not be substituted for the cohomology condition here.

The Yoneda lemma, applied to the representing Omega-spectrum spaces in each degree, makes the comparison more explicit. PutThe structure map of $D$ supplies $T_j\to T_{j+1}$, and $D$ is the homotopy colimit of spectra of this sequence. The inverse-tower transition on maps into $E$ is precomposition with $T_j\to T_{j+1}$. Under the suspension spectrum adjunction it takes $[D_{j+1},E_{j+1}{]}_{\ast }$ to $[D_j,E_j{]}_{\ast }$ by taking the adjoint through $D_j\to \Omega D_{j+1}$ and the Omega-spectrum identification $E_j\simeq \Omega E_{j+1}$.

Consequently the maps of represented cohomology theories are exactly the compatible degreewise operations:Only nonnegative degrees need be listed, since the suspension compatibility recovers the negative degrees. The Milnor exact sequence for maps of spectra gives the sharper descriptionThus $\mathrm{HPh}(D,E)\cong {\lim }_j^1[\Sigma D_j,E_j{]}_{\ast }$, with bonding maps coming from the same telescope. For an inverse tower $A_j$ with maps $r_j:A_{j+1}\to A_j$, $\lim A_j$ is the kernel and ${\lim }^1{A}_j$ is the cokernel ofThe first comparison quotients by spectrum homotopy; the second quotients by hyperphantom maps. These are different identifications.