Linear Algebra II HT23, Gram-Schmidt process

Created: February 06, 2023 | About these notes | View in context | Study these flashcards

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The Gram-Schmidt process takes in a basis $[u _ 1, \ldots u _ n]$ and outputs an orthonormal basis $[w _ 1, \ldots, w _ n]$. What additional property does this orthonormal basis have?
\[\text{span}(u _ 1, \ldots, u _ j) = \text{span}(w _ 1, \ldots, w _ j)\]

for all $j \le n$.

How could you prove every finite dimensional inner product space has an orthonormal basis?

Use the Gram-Schmidt process.

Say you’ve been given a basis $[u _ 1, \ldots, u _ n]$ to construct an orthonormal basis from and have made it as far as $[w _ 1, \ldots, w _ {j-1}]$ where $\text{span}(u _ 1, \ldots, u _ {j-1}) = \text{span} (v _ 1, \ldots, v _ {j-1})$. What is the expression for $w _ j$ (the unnormalised basis vector), and then $v _ j$ (the normalised one)?
\[w _ j = u _ j - \sum^{j-1} _ {k=1} \langle u _ j , v _ k \rangle v _ k\]

and

\[v _ j = \frac{w _ j}{ \vert \vert w _ j \vert \vert }\]

What’s the main proof technique behind proving the validity of the Gram-Schmidt process, i.e.

Given $v _ 1, \ldots, v _ k$ lineraly independent vectors, then there exists $w _ 1, \ldots, w _ k$ such that:

  1. $w _ i$ are mutually orthogonal
  2. $\text{span}(w _ 1, \ldots, w _ i) = \text{span}(v _ 1, \ldots, v _ i)$

for each $i \le k$.

?

Proof by induction.

When proving the validity of the Gram-Schmidt process, i.e. that

Given $v _ 1, \ldots, v _ k$ lineraly independent vectors, then there exists $w _ 1, \ldots, w _ k$ such that:

  1. $w _ i$ are mutually orthogonal
  2. $\text{span}(w _ 1, \ldots, w _ i) = \text{span}(v _ 1, \ldots, v _ i)$

for each $i \le k$.

you proceed by induction. At some stage in the proof, you have to show (assuming the inductive hypothesis) that

\[\text{span}(v _ 1, \ldots, v _ {k-1}, v _ k) = \text{span}(w _ 1, \ldots, w _ {k-1}, w _ k)\]

Can you justify this?
\[\begin{aligned} \text{span}(v _ 1, \ldots, v _ {k-1}, v _ k) &= \text{span}(v _ 1, \ldots, v _ {k-1}, v _ k) \\\\ &= \text{span}(w _ 1, \ldots, w _ {k-1}, w _ k) \end{aligned}\]

Step one by the inductive hypothesis, and step two by the Steinitz exchange lemma.

Proofs

Prove (via the Gram-Schmidt process) that given $v _ 1, \ldots, v _ k$ lineraly independent vectors, then there exists $w _ 1, \ldots, w _ k$ such that:

  1. $w _ i$ are mutually orthogonal
  2. $\text{span}(w _ 1, \ldots, w _ i) = \text{span}(v _ 1, \ldots, v _ i)$

for each $i \le k$.

Todo.



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