The Gray Filtration on Phantom Maps

This paper is a study of the Gray index of phantom maps. We give a new, tower theoretic, definition of the Gray index, which allows us to study the naturality properties of the Gray index in some detail. McGibbon and Roitberg have shown that if f∗ is surjective on rational cohomology, then the induced map on phantom sets is also surjective. We show that if f ∗ is surjective just in dimension k, then f induces a surjection on a certain subquotient of the phantom set. If the condition holds for all k, we recover McGibbon and Roitberg’s theorem. There is a dual result, and a theorem on phantom maps into spheres which holds one dimension at a time as well. Finally, we examine the set of phantom maps whose Gray index is infinite. The main theorem is a partial verification of our conjecture that if X and Y are nilpotent and of finite type, then every phantom map f : X −→Y must have finite index. Math. Subject Classifications: 55S37, 55P30, 55P62, 20G10, 20J05