Prove: Every Subfield of C Contains Rational Numbers

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Discussion Overview

The discussion revolves around the exercise of proving that every subfield of the complex numbers (C) contains all rational numbers (Q). Participants explore the properties of fields, specifically focusing on closure under operations and the implications of field characteristics.

Discussion Character

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

Main Points Raised

  • One participant questions whether the set {0,1} satisfies the conditions to be a field.
  • Another participant emphasizes the requirement of closure under operations for a set to be a field.
  • A participant suggests that to have closure under addition and subtraction, the integers (Z) must be included, and for multiplication and division, the rational numbers (Q) are necessary.
  • There is a discussion about fields with characteristic 2, where one participant proposes that such fields could exist, leading to further questions about characteristics of fields.
  • Another participant asserts that any subfield of C must contain Q, reasoning that if 1 and 0 are elements, then all natural numbers must also be elements, leading to the inclusion of integers and rational numbers.
  • Participants express uncertainty regarding whether the characteristic of a field is 1 or 2, and seek clarification on the definitions involved.
  • One participant notes that C is a field of characteristic 0, implying that its subfields must also share this characteristic.
  • There is a clarification that the set {0,1} is a field with characteristic 2 but is not a subfield of C.

Areas of Agreement / Disagreement

Participants generally agree that any subfield of C must contain Q, but there is disagreement regarding the implications of field characteristics and whether certain sets can be considered subfields of C.

Contextual Notes

Participants discuss the definitions and properties of fields, including closure under operations and the implications of field characteristics, but do not resolve the nuances of these concepts.

poolwin2001
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I am self studying linear algebra from `Linear Algebra' by Hoffman and Kunze.
One of exercise Q is:
Prove that Every subfield F of C contains all rational numbers.

But doesn't the set {0,1}(with the usual +,-,.) satisfy all conditions to be a field?
 
Last edited:
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what's 1+1?
 
As Matt implies, closure under the operations is a requirement.
 
EEK!I forgot about 1+1 :(
to have closure under addn & subtr you need to have Z.
to have closure under multiplication and division(or existence of x^-1 for all x) you need Q.Therefore All subfields of C should have atleast Q in them.
Is my proof correct?
 
Wait a minute!
My set can be a field with characteristic 2 (1+1=0).(or is it characteristic 1)
Which brings me to the next Question.
P.T. All zero characteristic fields contain Q.
Any hints how to begin?
Thanks is advance
 
Yes that's basically correct:

1 and 0 must be elements (actually I am a little unclera on this is the trivial field technically a subfield of C?) thus any 1+1+1...+1 is also an element so all the natural nunmbers must be elements and by additve inverse all integers must be elements. Any number in Q can be given by n*1/m where n and m are integers (m not equal to zero), by muplicative inverse 1/m must be in the any subfield of C, therefore any subfield of C has Q as a subfield.
 
Thanks .but is the charactesitic 1 or 2?
 
Please don't give away the whole ans.Just gimme a hint.Thanks anyway
 
poolwin2001 said:
Wait a minute!
My set can be a field with characteristic 2 (1+1=0).(or is it characteristic 1)
Which brings me to the next Question.
P.T. All zero characteristic fields contain Q.
Any hints how to begin?
Thanks is advance

Yes, but it's not a subfield of C though is it.

Just look at the definition of a field with charestic 0.
 
  • #10
True
C is a zero charecteristic field so are its subfields thereof
Why?
 
  • #11
because of the prefix sub. If it is a subfield then adding two elements in the subfield must give the same answer as adding them in the field, so if 1+1..+1=0 in the subfield, it equals zero in the field and hence the field has characteristic p for soem prime.


All fields must contain 0 and 1 and these are distinct (so the set {0} with addition and multiplication isn't a field, jcsd), so all fields of char 0 contain a copy of Q. The proof is the same as for the large field being C. You didn't actually use anything other than it was a field of characteristic zero did you?
 
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  • #12
So {0,1,+,.} is a field with charecteristic 2.But it is not a subfield of C.
Thanks.
 

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