A fundamental fact about Linear Algebra

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Discussion Overview

The discussion revolves around a challenge problem in linear algebra concerning the properties of vector spaces over infinite fields. Specifically, the problem asks participants to prove that a vector space cannot be expressed as a set-theoretic union of a finite number of proper subspaces.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant presents a proof by contradiction, assuming that a vector space $V$ can be expressed as a union of $n$ proper subspaces and deriving a contradiction based on the properties of infinite sets.
  • The proof involves constructing a set $S$ from linear combinations of elements from the proper subspaces, leading to a contradiction with the assumption of distinct elements in the same subspace.
  • Another participant notes the lack of responses and suggests allowing more time for others to engage in the discussion.
  • A subsequent post reiterates the suggestion to wait longer for participation, indicating a desire for more interaction in the thread.

Areas of Agreement / Disagreement

Participants do not appear to have reached a consensus on the problem, as there is only one proof presented and no additional contributions or challenges to the proof have been made. The discussion remains largely unresolved due to the lack of engagement.

Contextual Notes

There is an absence of further exploration or critique of the proof provided, and no alternative solutions or viewpoints have been introduced. The discussion is limited by the lack of participation.

Who May Find This Useful

Individuals interested in linear algebra, particularly those exploring properties of vector spaces and proof techniques in mathematics, may find this discussion relevant.

caffeinemachine
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MHB
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Hello MHB,
This is probably my first challenge problem which falls in the 'University Math' category.

$V$ is a vector space over an infinite field $F$, prove that $V$ cannot be written as a set theoretic union of a finite number of proper subspaces.
 
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Nobody participated :(

Here's my solution:

Assume contradictory to the problem. Let $n$ be the minimum integer such that $V$ can be written as $V=V_1\cup\cdots\cup V_n$ where each $V_i$ is a proper subspace of $V$. Thus, \begin{equation*}\forall i,\exists x_i\in V \text{ such that } x_i\in V_j\iff j=i\tag{1}\end{equation*}Now consider $S=\{f_1x_1+\cdots+f_nx_n:f_i\in F\}$. Clearly this set is infinite, thus, by PHP, there is a $k$ such that $a,b\in V_k$ for distinct $a$ and $b$ in $S$. This contradicts $(1)$. Hence we achieve the required contradiction and the proof is complete.
 
caffeinemachine said:
Nobody participated :(

I would give everyone at least a week to participate, since not everyone checks in on a daily basis. Some may only have time once a week.
 
MarkFL said:
I would give everyone at least a week to participate, since not everyone checks in on a daily basis. Some may only have time once a week.
My bad then. Next time I'll wait for a week.
 

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