# Finding the orthogonal complement of a span?

Let $$V = P_3(\mathbb{R})$$ the vector space of all polynomials in $$t$$ of degree at most 3.

$$W = M_{2\times 2}(\mathbb{R})$$ the vector space of all $$2\times 2$$ real matrices.

Define $$T:V \rightarrow W$$ by $$T(a_1 + a_2t + a_3t^2 + a_4t^3) = \begin{bmatrix} \dfrac{1}{\sqrt{1}}a_1 & \dfrac{1}{\sqrt{2}}a_2 \\ \dfrac{1}{\sqrt{4}}a_4 & \dfrac{1}{\sqrt{3}}a_3 \end{bmatrix}$$.

My question:

1. Let $$U = span\left(\left\{\begin{bmatrix}1 & 0\\ 0 & 1\end{bmatrix}, \begin{bmatrix}1 & 2\\ 0 & 3\end{bmatrix}\right\}\right)$$, find $$U^\bot$$.

Is my attempt correct?

Using Gram-schmidt process to get a set of orthonormal basis of $$M_2$$ with $$u_1, u_2$$ as given.

Denoting $$M$$ as $$a_{11}E_{11} + a_{12}E_{12} + a_{21}E_{21} + a_{22}E_{22}$$, and denoting 2 matrices given as column vectors $$(\alpha_1, \alpha_2)$$, as the new matrtix $$A$$, solve $$A'x = 0$$ and orthonormalize the new basis, we get: \begin{align*} A = \begin{pmatrix} 1 & 1\\ 0 & 0\\ 0 & 2\\ 1 & 3 \end{pmatrix} \end{align*} Performing elementary row operations on $$\begin{bmatrix} 1 & 0 & 0 & 1\\ 1 & 0 & 2 & 3 \end{bmatrix}$$, we get the RREF: \begin{align*} \begin{bmatrix} 1 & 0 & 1 & 1\\0 & 0 & 1 & 0 \end{bmatrix} \end{align*} To find the null space, we solve the matrix equation \begin{align*} \begin{bmatrix} 1 & 0 & 0 & 1\\ 0 & 0 & 1 & 0 \end{bmatrix} \begin{bmatrix} x_1\\x_2\\x_3\\x_4 \end{bmatrix} = \begin{bmatrix} 0\\0 \end{bmatrix} \end{align*} We take $$x_2 = 2$$, $$x_4 = s$$, then $$x_1 = -s$$, $$x_3 = 0$$. Hence, \begin{align*} \vec{x} = \begin{bmatrix} -s\\t\\0\\s \end{bmatrix} = \begin{bmatrix} 0\\1\\0\\0 \end{bmatrix} t + \begin{bmatrix} -1\\0\\0\\1 \end{bmatrix}s \end{align*} By solving $$A^Tx = 0$$, the orthogonal complement $$U^\bot$$ is: \begin{align*} \left\{\begin{bmatrix} 0\\1\\0\\0 \end{bmatrix},\begin{bmatrix} -1\\0\\0\\1 \end{bmatrix} \right\} \end{align*}

• What has the first sentence to do with the rest of the question? Commented Jun 18, 2021 at 13:57
• I'm not exactly sure. The problem starts with the first sentence, then a series of sub problems follow. I omitted the other sub-problems. In my attempt, I did not use any information from the first sentence.
– muw
Commented Jun 18, 2021 at 13:59
• In order to talk about $U^\perp$ for $U\subset M_{2\times 2}(\mathbb R)$ you need to specify an inner product on $M_{2\times 2}(\mathbb R)$ first. Please edit your post to include that information. Commented Jun 19, 2021 at 12:30
• If you look to math.stackexchange.com/q/4144237/715008 the question is similar but the inner product is defined. What you need is inner product for W. The inner product you have added is for V and does not relate to this exercise Commented Jun 19, 2021 at 16:15
• If you only want to find $U^\perp$ in $M_{2\times 2}(\mathbb R)$, why do you even mention the space $V$? It is completely irrelevant to the problem. Commented Jun 20, 2021 at 15:58

$$u\in U\Rightarrow u=\alpha\cdot \begin{bmatrix}1&0\\0&1\end{bmatrix}+\beta\cdot\begin{bmatrix}1&2\\0&3\end{bmatrix}=\begin{bmatrix}\alpha+\beta&2\beta\\0&\alpha+3\beta\end{bmatrix}=\begin{bmatrix}a_1&\frac{a_2}{\sqrt{2}}\\\frac{a_4}{2}&\frac{a_3}{\sqrt{3}}\end{bmatrix}, \alpha, \beta \in \mathbb R$$

$$\begin{array}{rrlrlr}u\rightarrow p&=&a_1+a_2t+a_3t^2+a_4t^3\\p&=&(\alpha+\beta)+2\sqrt{2}\beta t+\sqrt{3}(\alpha+3\beta)t^2\\ q\in V, q&=&x_1+x_2t+x_3t^2+x_4t^3 \end{array}$$

$$=0\Rightarrow (\alpha+\beta)x_1+\sqrt{2}\beta x_2+\frac{(\alpha+3\beta)x_3}{\sqrt{3}}=0$$

$$\alpha\left(x_1+\frac{x_3}{\sqrt{3}}\right)+\beta(x_1+\sqrt{2}x_2+\sqrt{3}x_3)=0$$

$$x_1=s, x_3=-s\sqrt{3}, x_2=\sqrt{2}s, x_4=r, s, r\in \mathbb R$$

$$q=s+\sqrt{2}st-s\sqrt{3}t^2+rt^3$$

$$T(q)=\begin{bmatrix}s&s\\\frac{r}{2}&-s\end{bmatrix}=s\begin{bmatrix}1&1\\0&-1\end{bmatrix}+r\begin{bmatrix}0&0\\1&0\end{bmatrix}$$

$$U^\bot=span\left(\begin{bmatrix}1&1\\0&-1\end{bmatrix},\begin{bmatrix}0&0\\1&0\end{bmatrix}\right)(\therefore)$$

• The exercise is incomplete without the-now removed - definition of inner product: $$a,b\in V:<a,b>=\int_0^1{a_1b_1+a_2b_2t+a_3b_3t^2+a_4b_4t^3dt}$$ Commented Jun 21, 2021 at 3:10
• Some people say that that is irrelevant in the problem. So it is relevant?
– muw
Commented Jun 21, 2021 at 4:10
• Up until now the OP did not specify an inner product on $W$. Without it, you can't compute orthogonal complements in $W$. Commented Jun 21, 2021 at 7:09
• What do you mean? He indeed placed the one thing I removed.
– muw
Commented Jun 21, 2021 at 16:02
• As I said: You never gave a definition of an inner product on $M_{2\times 2}(\mathbb R)$. Commented Jun 21, 2021 at 16:23