What Causes the Error in Calculating the L2 Norm of Complex Functions?

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Discussion Overview

The discussion revolves around the calculation of the L2 norm of complex functions and the potential errors in deriving the expression for \|f-jg\|^2. Participants explore the definitions and properties of inner products in the context of complex functions, examining assumptions and steps taken in the calculations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant attempts to derive the expression \|f-jg\|^2 = \|f\|^2 - 2 \Im\{\} but questions where their assumption may be incorrect.
  • Another participant asks for clarification on whether f and g are complex functions or just their real parts, suggesting that the expression may only hold for real parts.
  • A different participant points out a potential error in the integration step and questions the necessity of using the inner product definition, implying that it may be more straightforward to apply known properties of inner products.
  • One participant expresses confusion about obtaining an imaginary part in their calculations, leading to a discrepancy in the expected result involving the imaginary part of the inner product.
  • A question is raised regarding the inner product involving a complex scalar, specifically and , indicating a need for clarification on the properties of inner products in complex vector spaces.
  • Another participant challenges the correctness of the steps taken in the calculations, prompting a discussion on the properties of inner products in complex spaces.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions made regarding the nature of the functions involved and the validity of the steps taken in the calculations. There is no consensus on the correct approach or resolution of the errors identified.

Contextual Notes

Participants highlight potential limitations in their assumptions about the functions being complex or real, as well as the implications of using the inner product definition. The discussion remains open regarding the properties of inner products in complex vector spaces.

divB
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Hi,

I want to show:

<br /> \|f-jg\|^2 = \|f\|^2 - 2 \Im\{&lt;f,g&gt;\} + \|g\|^2<br />

However, as far as I understand, for complex functions &lt;f,g&gt; = \int f g^* dt, right? Therefore:

<br /> \|f-jg\|^2 = &lt;f-jg, f-jg&gt; = \int (f-jg)(f-jg)^* dt = \int (f-jg)(f+jg) dt = \int f^2 + jfg - jfg + g^2 dt = \|f\|^2 + \|g\|^2<br />

Where is my wrong assumption?
Thanks.
 
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Are f and g complex functions? Then you should write f = Re(f) + j Im(f), g = Re(g) + j Im(g).

Or are they just the real parts of a single function? Because then you've just shown that ||f||² = ||Re f||² + ||Im f||², which makes sense, right?
 
divB said:
<br /> \int (f-jg)(f-jg)^* dt = \int (f-jg)(f+jg) dt<br />
This one should be $$\int (f-jg)(f-jg)^* dt = \int (f-jg)(f^*+jg^*) dt.$$ But why use the definition of <,> at all? I assume that you have already proved that it's an inner product. So why not just use that?
 
Hi, thank you. Ok, no integrals, but only use <,>

I am again confused :(

\|v\|^2 = &lt;v,v&gt;, as far as I understand also for complex functions. But then, with using only the inner product, I have no chance to obtain an imaginary part only:

<br /> \|f-jg\|^2 = &lt;f-jg,f-jg&gt; = &lt;f,f-jg&gt;-&lt;jg,f-jg&gt; \\<br /> = &lt;f,f&gt; - &lt;f,jg&gt; - (&lt;jg,f&gt;-&lt;jg,jg&gt;) \\<br /> = &lt;f,f&gt; - &lt;f,jg&gt; - &lt;jg,f&gt; + &lt;jg,jg&gt; \\<br /> = &lt;f,f&gt; - j&lt;f,g&gt; - j&lt;g,f&gt; - &lt;g,g&gt;<br /> = \|f\|^2 - 2j&lt;f,g&gt; - \|g\|^2<br />

But 2j&lt;f,g&gt; is not 2\Im&lt;f,g&gt;...
 
What is <cf, g> and what is <f, cg> if c is a complex number?
 
divB said:
&lt;f,f&gt; - &lt;f,jg&gt; - &lt;jg,f&gt; + &lt;jg,jg&gt; \\<br /> =&lt;f,f&gt; - j&lt;f,g&gt; - j&lt;g,f&gt; - &lt;g,g&gt;\\<br /> = \|f\|^2 - 2j&lt;f,g&gt; - \|g\|^2<br />
These steps are both wrong. What are the properties of an inner product on a complex vector space?
 
Last edited:

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