Solution

ID: past-exam-of-the-topics-in-applied-mathematics-course-of-the-university-of-oxford/2009/a/1/c/i/solution

For , the second-iterate equation is equivalent to . Write and let paired points be . The condition becomes . At , , so strict concavity of the positive hyperbolic tangent gives one nonzero positive , as well as . Here it is exactly .
Thus the four nonnegative fixed points of are
Zero has periodic-orbit multiplier and the central point has periodic-orbit multiplier , so both are unstable. The outer two points are exchanged by , since and . Their common second-iterate periodic-orbit multiplier is
Its magnitude is below one, so both outer fixed points of are stable. This verifies the crossings and stability in the supplied graph rather than reading stability from crossing direction alone.

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