I cannot seem to figure out how to prove the following:

"Prove that if a vector space has dimension $n$ then any $n + 1$ of its vectors are linearly dependent."

I reckon applying proof by contradiction might be a useful approach, but cannot seem to figure it out. Maybe weak induction as well? How would one go about this?

Below Edit 1 - Made to accomodate question requesting further specification: 

"Linearly independent": A set of vectors $\{a_1, a_2,a_3...,a_n\}$ is seen as being linearly independent if $x_1a_1 + x_2a_2 + x_3a_3 + x_na_n = 0 \space (null vector)$ only is satisfied for $x_1 = x_2 = x_3 = x_n = 0$, then the set $\{a_1, a_2,a_3...,a_n\}$ is linearly independent. (${x_1,x_2,x_3,...,x_n}$ are arbitrary scalars)

"Definition of dimension": Let $V$ be a vector space. The number of vectors in a basis for $V$ is called the dimension of $V$, and is written $ \dim V$.

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    $\begingroup$ What is your definition of "linearly independent" (there are many equivalent definitions, so it is important that we know which one(s) you are allowed to use in your proof). $\endgroup$ – kccu Apr 1 '19 at 19:42
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    $\begingroup$ This answer to this question probably depends on the definition of dimension you have. $\endgroup$ – Bernard Apr 1 '19 at 19:42
  • $\begingroup$ Perhaps contrapositive is best. If you have $n+1$ linearly independent vectors in a vector space, either they span or they do not. What do you conclude about the dimension? $\endgroup$ – Ted Shifrin Apr 1 '19 at 20:15
  • $\begingroup$ Hi @kccu, I appreciate your interest. I have hereby specified the term linearly independent as I was not aware of the fact that the term had multiple definitions. Thank you. $\endgroup$ – ms99 Apr 1 '19 at 20:15
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    $\begingroup$ There is a nice proof at math.harvard.edu/archive/23b_spring_04/hw3sc.pdf on page 3, proof 3 that proves this. $\endgroup$ – user648059 Apr 1 '19 at 22:09

For sake of contradiction, suppose there is a set of vectors $\{u_1, \ldots, u_{n+1}\}$ in an $n$-dimensional vector space $V$ such that $\{u_1, \ldots, u_{n+1}\}$ are linearly independent. Let $B = \{v_1, \ldots, v_n\}$ be a basis for $V$. Then $B$ spans $V$, and we can write

$$ u_1 = a_1v_1 + \cdots + a_nv_n $$

Since $\{u_1, \ldots, u_{n+1}\}$ is linearly independent, it follows that no $u_i$ can be zero. This implies that there is at least one $j$ such that $a_j \neq 0$. Without loss of generality, assume that $j = 1$. Then we may write

$$ \begin{aligned} v_1 & = \frac{1}{a_1}u_1 - \frac{a_2}{a_1}v_2 - \cdots - \frac{a_n}{a_1}v_n & (1) \end{aligned} $$

Now let $B_1 = \{u_1, v_2, \ldots, v_n\}$. Since $B$ spans $V$, we can write

$$ v = \lambda_1v_1 + \cdots + \lambda_nv_n $$

for any $v\in V$. By $(1)$, we have

$$ \begin{align} v & = \lambda_1(\frac{1}{a_1}u_1 - \frac{a_2}{a_1}v_2 - \cdots - \frac{a_n}{a_1}v_n) + \lambda_2v_2 + \cdots + \lambda_nv_n \\ & = \frac{\lambda_1}{a_1}u_1 + \frac{-\lambda_1 a_2}{a_1}v_2 + \lambda_2v_2 + \cdots + \frac{-\lambda_1 a_n}{a_1}v_n + \lambda_nv_n \\ & = \frac{\lambda_1}{a_1}u_1 + (\frac{-\lambda_1 a_2}{a_1} + \lambda_2)v_2 + \cdots + (\frac{-\lambda_1 a_n}{a_1} + \lambda_n)v_n \\ & = \lambda_1^{'}u_1 + \lambda_2^{'}v_2 + \cdots + \lambda_n^{'}v_n \\ \end{align} $$

Thus, we can write any $v \in V$ in terms of the elements of $B_1$, which means $B_1$ spans $V$.

Suppose we have obtained $B_{i - 1} = \{u_1,\ldots, u_{i-1},v_i,\ldots, v_n\}$ and have shown that it spans $V$. Then we may write

$$ u_i = a_1u_1 + \cdots + a_{i-1}u_{i-1} + a_iv_i + \cdots + a_nv_n $$

for some $a_1, \ldots, a_n \in \mathbb{R}$. Since $u_i$ is a non-zero vector, there must be a $k$ such that $a_k \neq 0$. Let $j$ be the largest index for which $a_j \neq 0$. This $j$ must satisfy $j \geq i$, for if $j < i$, then $a_i = a_{i+1} = \ldots = a_n = 0$, which implies that

$$ u_i = a_1u_1 + \cdots + a_{i-1}u_{i-1} $$

which contradicts that $\{u_1, \ldots, u_{n+1}\}$ is linearly independent. Without loss of generality, assume $j = i$. Then we can switch $u_i$ with $v_i$ in $B_{i-1}$ to obtain $B_i = \{u_1,\ldots, u_{i-1}, u_i,v_{i+1},\ldots, v_n\}$, which can be shown to span $V$ by a substitution similar to the one using $(1)$ above.

Continue to do this until the $n$th step, where $B_n = \{u_1, u_2, \ldots, u_n\}$. Previously, we showed that if $B_{i-1}$ spans $V$, then our operation of switching $u_i$ with $v_i$ to get $B_i$ also makes $B_i$ span $V$. Therefore, $B_n$ must span $V$ (by induction starting with $B_1$). Since $u_{n+1}$ is in $V$, and since $B_n$ spans $V$, we can write

$$ u_{n+1} = a_1u_1 + \cdots + a_nu_n $$

for some $a_1, \ldots, a_n \in \mathbb{R}$. But this contradicts that $\{u_1, \ldots, u_{n+1}\}$ is linearly independent. Therefore, it must not be the case that $\{u_1, \ldots, u_{n+1}\}$ is linearly independent, which is to say that $\{u_1, \ldots, u_{n+1}\}$ must be linearly dependent. $$\tag*{$\blacksquare$}$$


If there are $n+1$ linearly independent vectors, then $\operatorname{dim} V\ge n+1\gt n \Rightarrow \Leftarrow$.

(This is the contrapositive, as @Ted Shifrin suggested. )


Let $n$ be the dimension of $V$. Let $u_{i}\in V$ for $i=1$ to $n{+}1$ be a set of linearily independent set. Hence, $u_{i}$ for $i{=}1$ to $n$ are also linearly independent, hence $u_{i}$ for $i=1$ to $n$ is a basis set for $V$ (you need to prove that let $V'= \text{span}\{u_{1},u_{2}\cdots u_{n}\}$, then $V'\subseteq V$ and $\text{dim}(V')=\text{dim}(V)=n$, then $V'=V$, see this ) . Since $u_{n+1}\in V$, thus it has a unique representation using the basis vectors; thus

$$ u_{n+1} = \sum_{i=1}^{n}\alpha_{i}u_{i} $$

Hence, $u_{i},\ i\in 1\cdots n{+}1$ are linearly dependent.


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