Does Every Field Have a Subfield Isomorphic to Q or Z mod p?

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Discussion Overview

The discussion revolves around whether every field has a subfield that is isomorphic to either the rational numbers (Q) or the integers modulo a prime (Z mod p). Participants explore the implications of field characteristics and the concept of prime subfields.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant suggests that every field contains the subfield {0,1}, questioning if this makes the statement true, albeit vacuously.
  • Another participant introduces the concept of the prime subfield, stating it is the intersection of all subfields and is isomorphic to Q or Z_p based on the field's characteristic.
  • A later reply provides a reasoning process involving the sum of 1s in a field, concluding that if the sum equals zero, the subring generated by 1 is isomorphic to Zn, with n being prime in the context of fields.
  • It is noted that if n is never zero for all positive integers, the subring generated by 1 is isomorphic to the integers, leading to the inclusion of Q in fields containing the integers.
  • One participant clarifies that {0,1} is not a field unless 1+1=0, which adds a condition to the earlier claims.

Areas of Agreement / Disagreement

Participants present various viewpoints and reasoning regarding the existence of subfields isomorphic to Q or Z mod p, but no consensus is reached on the initial claim or its implications.

Contextual Notes

The discussion involves assumptions about field characteristics and the definitions of subfields and prime subfields, which may not be universally agreed upon or fully explored.

ECmathstudent
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I just wrote an exam in Algebra II, and one of our true or false questions got me thinking. Does every field have a subfield which is isomorphic to the Q or Z mod p? I put it as true, for the wrong reasons, vaguely remembering a similar statement about integral domains and mixing it up. So, after the exam I was sure I was wrong.
But doesn't every field have the subfield {0,1}? Wouldn't this make the statement true, if somewhat vacuously?
 
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it's called the prime subfield, which is the intersection of all subfields of the given field. it's isomorphic to Q or Z_p depending on whether the field has characteristic 0 or p
 
Oh, thanks. I don't have a book anymore, when I tried to look it up on google I wasn't really getting any results.
 
yes.

it is quite easy to show this.

suppose 1+1+...+1 (n times) = 0.

then the subring generated by 1 is isomorphic to Zn.

however, since F is a field, none of the elements:

1, 1+1, 1+1+1, etc. can be a zero-divisor, which forces n to be prime.

on the other hand, suppose that n1 is never 0, for all n in Z+.

then the subring generated by 1 is isomorphic to the integers, and

any field containing the integers also contains the field of quotients of the integers, which is Q.

(also {0,1} is not a field, unless 1+1 = 0).
 

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