Why Does Complex Mapping T(z) = Az + B Fail Linearity?

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Discussion Overview

The discussion revolves around the nature of the complex mapping T(z) = Az + B, specifically addressing why it fails to satisfy the conditions of linearity. Participants explore the implications of the mapping being affine rather than linear and consider its effects on geometric transformations in the complex plane.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant questions the linearity of the mapping T(z) = Az + B, suggesting that it does not satisfy the linearity condition T(a * z1 + z2) = a * T(z1) + T(z2).
  • Another participant clarifies that the mapping is not linear unless B=0, categorizing it as an affine map instead.
  • A different participant references a source, K. Stroud's Advanced Engineering Mathematics, noting that the transformation maps lines in the complex plane to lines in the w-plane, but expresses uncertainty about its applicability to other shapes.
  • One participant elaborates on the differences between real-valued and complex-valued linear maps, indicating that complex mappings involve both scaling and rotation, and that complex lines correspond to real planes in a two-dimensional context.

Areas of Agreement / Disagreement

Participants generally agree that T(z) = Az + B is not a linear map due to the presence of B, but there is some disagreement regarding the implications of this classification and its effects on geometric transformations.

Contextual Notes

The discussion highlights potential variations in definitions of linear maps across different texts, as well as the specific conditions under which the mapping behaves linearly or affine. There is also uncertainty regarding the transformation's effects on shapes beyond lines.

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Hello,

Given the complex linear mapping: T(z) = Az + B where A is real and B is complex. However trying to show that T(a * z1 + z2) = a * T(z1) + T(z2) does not work which implies the mapping is not linear? Why does not this rule apply here?

Thanks.
 
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The map you posted is not a linear map. It's only linear if ##B=0##. The map you posted is called an affine map.

That said, I can probably imagine some books which define "linear map" somewhat different than usual.
 
I see now. Well I found this in the K. Stroud's Advanced Engineering Mathematics on complex analysis. The same transformation when applied to a line, the image is another line in the w-plane, but not sure if the same applies to any other shape.
 
Note that while a Real-valued linear map y=kx only scales , a complex-valued linear map both scales and rotates (e.g., multiply using polars); Complex lines, being 2-dimensional, are Real planes: notice that , for fixed Complex a, the set { ##{az: z \in \mathbb C}##} is the entire complex plane .
 

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