Cardinality of Complex vs. Real

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    Cardinality Complex
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Discussion Overview

The discussion revolves around the cardinality of complex numbers compared to real numbers, exploring the relationships between these sets through bijections and injections. Participants examine the implications of number representations and ambiguities in decimal notation, particularly in the context of set theory and number theory.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes that complex numbers can be represented as ordered pairs of real numbers, suggesting a potential bijection between the sets.
  • Another participant questions whether the proposed mapping constitutes a one-to-one correspondence and highlights the need to address ambiguous representations of numbers.
  • Some participants assert that there is a one-to-one relationship between the reals and the Cartesian plane, and between complex numbers and the Cartesian plane, implying that their cardinalities are the same.
  • Concerns are raised about the ambiguity of numbers like 0.5 and 0.4999..., with some arguing that they lead to different results while others emphasize that they represent the same value.
  • A participant suggests that an injection from complex numbers to real numbers suffices for establishing cardinality, provided certain assumptions about decimal representations are made.
  • Another participant reiterates the ambiguity issue, stating that different representations of the same number can lead to confusion in the mapping process.

Areas of Agreement / Disagreement

Participants express differing views on the implications of number representation and the validity of the proposed mappings. There is no consensus on how to resolve the ambiguities or whether the proposed bijection is sufficient to establish cardinality.

Contextual Notes

Participants note limitations related to the representation of real numbers, particularly concerning finite and infinite decimals, and the implications of treating numbers like 0.5 and 0.4999... as equivalent or distinct.

Parth Dave
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Prove that the set of complex numbers has the same cardinality as the reals.

What I did was say that a + bi can be written as (a, b) where a, b belong to real. Which essentially means i have to create a bijection between (a, b) and z (where z belongs to real).

Suppose:
a = 0.a1a2a3a4a5...
b = 0.b1b2b3b4b5...

Then,

z = 0.a1b1a2b2a3b3...

Is there anything wrong with that?
 
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You need to do a little more work. First, does that constitute a one to one correspondence between the reals and the complex numbers? Also, you need to deal with ambiguous representations of certain numbers such as 0.5 and 0.4999...
 
Well essentially that shows that there is a one to one relationship between the reals and the cartesian plane (x, y). Also, there is a one to one relationship between complex and the cartesian plane. Thus, the cardinality for all 3 is the same.

Pertaining to the ambiguosity of certain number, I'm not sure if i see how they pose a problem because if x is 0.5 or 0.499999 you get a different result.

ps. pardon my ignorance. I just started set/number theory two days ago :-p .
 
Parth Dave said:
Well essentially that shows that there is a one to one relationship between the reals and the cartesian plane (x, y). Also, there is a one to one relationship between complex and the cartesian plane. Thus, the cardinality for all 3 is the same.

Pertaining to the ambiguosity of certain number, I'm not sure if i see how they pose a problem because if x is 0.5 or 0.499999 you get a different result.

The problem is that [itex]0.5[/itex] and [itex]0.4\bar 9[/itex] are the same number and lead to different results!
 
he only needs an injection from the complexes to the reals, since there is an inclusion the other way, and if he assumes his reals are finite or infinite decimals not ending in all 9's, then his map never sends any pair of reals to a decimal ending in all 9's. so he does get an injection.
 
Tide said:
The problem is that [itex]0.5[/itex] and [itex]0.4\bar 9[/itex] are the same number and lead to different results!

There is no real number 4.9999...
4.99... and 5 are just two metha-variables to denote the number 5 in R .Here denote means "interpretation." .
 
Last edited:

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