R^3/Kernel of 2x+3y-z/Orthonormal basis/Gram-Schmidt/Example

Let ${}V$ be the kernel of the linear mapping

$\mathbb {R} ^{3}\longrightarrow \mathbb {R} ,(x,y,z)\longmapsto 2x+3y-z.$

As a linear subspace of ${}\mathbb {R} ^{3}$, ${}V$ carries the induced inner product. We want to determine an orthonormal basis of ${}V$. For this, we consider the basis consisting of the vectors

$v_{1}={\begin{pmatrix}1\\0\\2\end{pmatrix}}\,\,{\text{ and }}\,\,v_{2}={\begin{pmatrix}0\\1\\3\end{pmatrix}}.$

We have ${}\Vert {v_{1}}\Vert ={\sqrt {5}}$; therefore,

${}u_{1}={\frac {v_{1}}{\Vert {v_{1}}\Vert }}={\begin{pmatrix}{\frac {1}{\sqrt {5}}}\\0\\{\frac {2}{\sqrt {5}}}\end{pmatrix}}\,$

is the corresponding normed vector. According to orthonormalization process, we set

${}{\begin{aligned}w_{2}&=v_{2}-\left\langle v_{2},u_{1}\right\rangle u_{1}\\&={\begin{pmatrix}0\\1\\3\end{pmatrix}}-\left\langle {\begin{pmatrix}0\\1\\3\end{pmatrix}},{\begin{pmatrix}{\frac {1}{\sqrt {5}}}\\0\\{\frac {2}{\sqrt {5}}}\end{pmatrix}}\right\rangle {\begin{pmatrix}{\frac {1}{\sqrt {5}}}\\0\\{\frac {2}{\sqrt {5}}}\end{pmatrix}}\\&={\begin{pmatrix}0\\1\\3\end{pmatrix}}-{\frac {6}{\sqrt {5}}}{\begin{pmatrix}{\frac {1}{\sqrt {5}}}\\0\\{\frac {2}{\sqrt {5}}}\end{pmatrix}}\\&={\begin{pmatrix}0\\1\\3\end{pmatrix}}-{\begin{pmatrix}{\frac {6}{5}}\\0\\{\frac {12}{5}}\end{pmatrix}}\\&={\begin{pmatrix}-{\frac {6}{5}}\\1\\{\frac {3}{5}}\end{pmatrix}}.\end{aligned}}$

We have

${}\Vert {w_{2}}\Vert =\Vert {\begin{pmatrix}-{\frac {6}{5}}\\1\\{\frac {3}{5}}\end{pmatrix}}\Vert ={\sqrt {{\frac {36}{25}}+1+{\frac {9}{25}}}}={\sqrt {\frac {70}{25}}}={\frac {\sqrt {14}}{\sqrt {5}}}\,.$

Therefore,

${}u_{2}={\frac {\sqrt {5}}{\sqrt {14}}}{\begin{pmatrix}-{\frac {6}{5}}\\1\\{\frac {3}{5}}\end{pmatrix}}={\begin{pmatrix}-{\frac {6}{\sqrt {70}}}\\{\frac {\sqrt {5}}{\sqrt {14}}}\\{\frac {3}{\sqrt {70}}}\end{pmatrix}}\,$

is the second vector of the orthonormal basis.