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**Unformatted text preview: **ASSIGNMENT 4 Â· SOLUTIONS MAT 572 A Â· FALL 2007 Problem 1 ( cf. [1, Exercise III.3.10]) . Find all MÂ¨ obius transformations which map the disc D = { z | | z | < 1 } onto itself. Solution. First, any such MÂ¨ obius transformation T must also take D c onto D c , since MÂ¨ obius transformations are invertible, and it follows that T takes the unit circle Î“ to itself. To see this, fix any z âˆˆ Î“, and choose sequences ( z n ) âŠ† D with z n â†’ z , and ( w n ) âŠ† D c with w n â†’ z . Then ( T ( z n )) âŠ† D and ( T ( w n )) âŠ† D c , so by continuity of T (and | Â· | ), we then have | T ( z ) | = | lim n T ( z n ) | = lim n | T ( z n ) | â‰¤ 1 â‰¤ lim n | T ( w n ) | = | lim n T ( w n ) | = | T ( z ) | , so | T ( z ) | = 1. So suppose T is a MÂ¨ obius transformation which takes Î“ to Î“, and write T ( z ) = az + b cz + d . If T maps D onto D , then it must take 0 to a point in the disc, so d 6 = 0; and T must also take âˆž to a point not in the disc. Thus, if also c 6 = 0, we must have (1.1) | T (0) | = b d < 1 < | T ( âˆž ) | = a c , and therefore | ad | > | bc | . If c = 0, then neither d nor a may be zero (otherwise...

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