What is the complete expansion of |1+a|^2 for a complex number a?

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Discussion Overview

The discussion revolves around the expansion of the expression |1+a|^2 for a complex number a, focusing on the mathematical formulation and the terms involved in the expansion. Participants explore the theoretical aspects of this expression without reaching a consensus on the complete expansion.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that |1+a|^2 can be expressed as 1+a+a*+..., where a* is the complex conjugate of a.
  • Another participant questions the completeness of this expansion, asserting that it is not an infinite series and provides a specific expansion involving real components x and y of a.
  • A different participant presents an alternative expansion, stating |1+a|^2 = (1+a)(1+a)* = 1 + a + a* + aa* and expresses confusion over the previous claims.
  • One participant acknowledges an error in their previous expansion, correcting it to include terms like 1 + 2x + x^2 + y^2.
  • Another participant critiques the earlier expansions, providing their own detailed breakdown of the expression, emphasizing the terms derived from the product of (1+a) and its conjugate.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the complete expansion of |1+a|^2, with multiple competing views and formulations presented throughout the discussion.

Contextual Notes

Some expansions presented rely on specific assumptions about the real and imaginary parts of the complex number a, and there are unresolved discrepancies in the terms included in the expansions.

cabrera
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Hi,

Could you help me to understand the following expansion I found in a book of qunatum mechanics.

|1+a|^2=1+a+a*+... where a* is the complex conjugate of a
 
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What are the remaining terms in the expansion? I don't think it is an infinite series...

If ##a=x+iy##, where ##x## and ##y## are real, then we have

\left| 1+a \right|^{2} = (1+a)(1+a)^{*}=(1+x+iy)(1+x-iy)=1+x+x^{2}+y^{2}=1+\frac{a}{2}+\frac{a^{*}}{2}+a^{*}a
 
hilbert2 said:
What are the remaining terms in the expansion? I don't think it is an infinite series...

If ##a=x+iy##, where ##x## and ##y## are real, then we have

\left| 1+a \right|^{2} = (1+a)(1+a)^{*}=(1+x+iy)(1+x-iy)=1+x+x^{2}+y^{2}=1+\frac{a}{2}+\frac{a^{*}}{2}+a^{*}a

I really don't see how you got this. To me it's

|1 + a|^2 = (1+a)(1+a)^* = (1 + a)(1 + a^*) = 1 + a + a^* + aa^*
 
^ Sorry, it should have been ##1+2x+x^{2}+y^{2}##. :blushing:
 
hilbert2 said:
What are the remaining terms in the expansion? I don't think it is an infinite series...

If ##a=x+iy##, where ##x## and ##y## are real, then we have

\left| 1+a \right|^{2} = (1+a)(1+a)^{*}=(1+x+iy)(1+x-iy)=1+x+x^{2}+y^{2}=1+\frac{a}{2}+\frac{a^{*}}{2}+a^{*}a


This expansion has errors...
If ##a=x+iy##, where ##x## and ##y## are real, then we have

\left| 1+a \right|^{2} = (1+a)(1+a)^{*}=(1+x+iy)(1+x-iy)=(1+x)^2+y^2=x^2+2x+1+y^2=1+x+iy+x-iy+(x+iy)(x-iy)=1+a+a{*}+(a)(a){*}
Hope this helps...
 

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