PERIODS FOR MAPS OF THE FIGURE-EIGHT SPACE
Let Per (f) denote the set of periods of all periodic points of a map f from a topological space into itself. Let 8 be the figure-eight space. We extend to the 8 the following theorem from the circle due to Block [1981]. Let [Formula: see text] be the circle. For every map [Formula: see text] with Per (f) ∩ {1, 2, …, n} = {1, n} and n > 2 we have Per (f) = {1, n, n+1, n+2, …}. Conversely, for every n ∈ ℕ with n > 2 there exists a map [Formula: see text] such that Per (f) = {1, n, n+1, n+2, …}. For the space 8 we prove the following. Let f: 8 → 8 be a continuous map having the branching point fixed and such that Per (f) ∩ {1, 2, …, n} = {1, n} with n > 4. Then Per (f) is either {1, n, n+1, n+2, …}, or {1, n, n+2, n+4, …} with n even, or {1, n, n+2, n+4, …}∪ {2n+2, 2n+4, 2n+6, …} with n odd. Conversely, for every n ∈ ℕ with n > 4, if A (n) is one of the above three subsets of ℕ, then there is a continuous map f: 8 → 8 having the branching point fixed and such that Per (f) = A (n).