Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2008/ib/paper-4/15c/solution
Past exam of the mathematics course of the University of Cambridge 2008 ib Paper 4 15C Solution by
Codex 0 Created 2026-09-24 Updated 2026-10-14
Choose the principal branch of the complex logarithm on , with . ThenThus and the imaginary part ranges over all real numbers. Conversely, for in the strip, is in and its principal logarithm is . Therefore is an analytic bijection, with analytic inverse . Every other continuous branch on the connected domain is for some fixed integer , so its image is the translated strip .
For , introduce the Möbius transformation , whose inverse is . Writing , its defining circle inequalities transform as follows:andHence maps bijectively onto the wedge . The principal branch of the complex logarithm is analytic throughout this wedge. Multiplying its logarithm by expands the interval of real parts to , and subtracting centers it at zero. The required conformal map is thereforeIts real part is , and its imaginary part takes every real value. More explicitly, its inverse on is obtained from and , which verifies both surjectivity and injectivity.
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