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Can you read my proof and tell me if it's correct? Thanks.

Let $V$ be a vector space over a complete topological field say $\mathbb R$ (or $\mathbb C$) with $\dim(V) = n$, base $e_i$ and norm $\|\cdot\|$. Let $v_k$ be a Cauchy sequence w.r.t. $\|\cdot\|$. Since any two norms on a finite dimensional space are equivalent, $\|\cdot\|$ is equivalent to the $l^1$-norm $\|\cdot\|_1$ which means that for some constant $C$, $\varepsilon > 0$, $k,j$ large enough, $$ \varepsilon > \|v_j - v_k\| \geq C \|v_j - v_k\|_1 e_i= C \sum_{i=1}^n |v_{ji} - v_{ki}| \geq |v_{ji} - v_{ki}|$$ for each $1 \leq i \leq n$. Hence $v_{ki}$ is a Cauchy sequence in $\mathbb R$ (or $\mathbb C$) for each $i$. $\mathbb R$ (or $\mathbb C$) is complete hence $v_i = \lim_{k \to \infty} v_{ki} $ is in $\mathbb R$ (or $\Bbb C$) for each $i$. Let $v = (v_1, \dots , v_n) = \sum_i v_i e_i$. Then $v$ is in $V$ and $\|v_k - v\| \to 0$:

Let $\varepsilon > 0$. Then $$ \|v_k - v\| \leq C \|v_k - v\|_1 = C \sum_{i=1}^n |v_{ki} - v_i| \leq C^{'}n \varepsilon$$

for $k$ large enough.

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Finite dimensional VS over $\mathbb{Q}$ are not complete, when equipped with, e.g. the Euclidean norm. –  user20266 Jul 8 '12 at 15:49
    
@Thomas Thank you. Is the proof correct now? (see edit) –  Rudy the Reindeer Jul 8 '12 at 15:53
    
Looks good! ${}$ –  Cameron Buie Jul 8 '12 at 16:02
    
@CameronBuie Thank you! –  Rudy the Reindeer Jul 8 '12 at 16:11
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The proof is correct, there's a litle detail that is incorrect, the first equality in the final equation should be an inequality and possibly a different constant $C'$ instead of $C$ depending on how you define these constants (you can get away with one constant.) –  Olivier Bégassat Jul 8 '12 at 16:31

1 Answer 1

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Yes, your proof is correct. Here, I will just reword it to slightly improve clarity and precision .

Let $V$ be a vector space over $\mathbb R$ (or $\mathbb C$) with $\dim(V) = n$ and norm $\|\cdot\|$. Let $\{e_i\}_{i=1,\cdots , n}$ be a base of $V$. Suppose $v_k$ be a Cauchy sequence w.r.t. $\|\cdot\|$.

Since any two norms on a finite dimensional space are equivalent, $\|\cdot\|$ is equivalent to the $l^1$-norm $\|\cdot\|_1$. So, there are $C,D>0$ such that, for all $w\in V$, $C \|w\|_1 \leq \|w\| \leq D \|w\|_1$.

So, we have, for all $\varepsilon > 0$, there is $N$ such that, if $k,j>N$, $$ \varepsilon > \|v_j - v_k\| \geq C \|v_j - v_k\|_1 = C \sum_{i=1}^n |v_{ji} - v_{ki}| \geq |v_{ji} - v_{ki}|$$ for each $1 \leq i \leq n$. Hence $v_{ki}$ is a Cauchy sequence in $\mathbb R$ (or $\mathbb C$) for each $i$. $\mathbb R$ (or $\mathbb C$) is complete hence $v_i = \lim_{k \to \infty} v_{ki} $ is in $\mathbb R$ (or $\Bbb C$) for each $i$. Let $v = (v_1, \dots , v_n) = \sum_i v_i e_i$. Then, it is clear that, $v$ is in $V$.

Let us prove $\lim_{k \to +\infty} \|v_k - v\| = 0$:

$$ \lim_{k \to +\infty} \|v_k - v\| \leq D \lim_{k \to +\infty} \|v_k - v\|_1 = D \lim_{k \to +\infty} \sum_{i=1}^n |v_{ki} - v_i| = D \sum_{i=1}^n \lim_{k \to +\infty} |v_{ki} - v_i|=0$$

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