Meaning of the square root and its contradiction

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Discussion Overview

The discussion centers around the definition and properties of the square root function, particularly in the context of complex numbers. Participants explore the implications of defining the square root function from the complex numbers to the real numbers and the resulting contradictions that arise, especially regarding the relationship between square roots and absolute values.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that the square root function should be defined as f: ℂ → ℝ⁺, while others argue it is actually f: ℂ → ℂ, noting that functions on complex numbers are not generally single-valued.
  • There is a discussion about the contradiction arising from the equation √(x²) = |x| and the case of √(-1) leading to different interpretations.
  • Some participants assert that √(x²) ≠ |x| for complex numbers, suggesting that the definition of absolute value for complex numbers plays a role in this distinction.
  • One participant mentions that the identity |x| = √(x̅x) is crucial for understanding the relationship between square roots and absolute values in the complex domain.
  • Concerns are raised about whether the definition of a function being single-valued is violated in the context of complex numbers.

Areas of Agreement / Disagreement

Participants do not reach consensus on the definition of the square root function or the implications of its properties in the complex number context. Multiple competing views remain regarding the nature of the square root and its relationship to absolute values.

Contextual Notes

The discussion highlights limitations in the definitions used and the assumptions about single-valued functions in complex analysis, but these remain unresolved.

kntsy
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Is square root a function in this way?

f:\mathbb C\rightarrow\mathbb R^+

However contradiction can be drawn:\sqrt{x^2}=|x|\text{ and } i^2=-1

\sqrt{-1}=\sqrt{i^2}=|i|=1
\sqrt{-1}=1 ??What is the problem in the definition or the deduction?
 
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kntsy said:
Is square root a function in this way?

f:\mathbb C\rightarrow\mathbb R^+
No, it's not- the square root function is from \mathbb C to \mathbb C. And functions, on the complex numbers, are not in general "single valued".

However contradiction can be drawn:


\sqrt{x^2}=|x|\text{ and } i^2=-1

\sqrt{-1}=\sqrt{i^2}=|i|=1
\sqrt{-1}=1 ??


What is the problem in the definition or the deduction?
The problem is with your definition, which is wrong.
 
HallsofIvy said:
No, it's not- the square root function is from \mathbb C to \mathbb C. And functions, on the complex numbers, are not in general "single valued".

That's a quite narrow view. It might not be the most mathematically useful way, but you might as well define Sqrt to be the principal value. Just as some people use Ln() instead of ln(). This makes writing out equations much easier because you have actual functions.

In this case the answer to the question is
\sqrt{x^2}\neq |x|
for complex numbers!
 
HallsofIvy said:
No, it's not- the square root function is from \mathbb C to \mathbb C. And functions, on the complex numbers, are not in general "single valued".
What does it mean for function not being single-valued? Does it violate the definition of function?
Gerenuk said:
In this case the answer to the question is
\sqrt{x^2}\neq |x|
for complex numbers!

Is it due to the definition of absolute value on complex number?
 
Is it due to the definition of absolute value on complex number?

I suppose you could say that. A very important identity for complex numbers is x \bar{ x } = | x |^2, so the correct identity relating square roots and absolute value (usually called modulus when extended to complex numbers) is | x | = \sqrt{ x \bar{ x } }. The reason that you're familiar with the identity | x | = \sqrt{ x^2 } for real numbers is that for a real number x, x=\bar{x}, so x\bar{x}=x^2.
 
The definition of "function" for real numbers requires that f(a) be a unique number. To use Gerunuk's phrase, that is too "narrow" a view for complex numbers.
 

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