Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2005/iii/paper-16/2/c/solution
Past exam of the mathematics course of the University of Cambridge 2005 iii Paper 16 2 c Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
Let have a dense forward orbit , and suppose that is surjective. Every tail is then dense. Indeed, continuity givesand the left side equals by surjectivity.
Given nonempty open sets , choose with . The dense tail supplies with . Taking gives . This proves the required positive-time intersection property.
The result can fail without surjectivity. Take the compact metric spacewith the metric induced from the real line. All nonzero points are isolated, so continuity there is automatic; at zero, proves continuity. The forward orbit of is , whose closure is , so this is point transitivity. But . The singletons and are nonempty open sets, and for every ,The isolated initial point can be visited only at the start of the dense forward orbit; deleting that initial segment destroys density.
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