User:GFauxPas/Sandbox

Welcome to my sandbox, you are free to play here as long as you don't track sand onto the main wiki. --GFauxPas 09:28, 7 November 2011 (CST)

We have $\rho\left({\mathbf A}\right) + \nu\left({\mathbf A}\right) = \text{number of columns} = 3$, which we might not have a page for.

From Null Space Contains Only Zero Vector iff Columns are Independent, $\nu\left({\mathbf A}\right) = 1$ iff the column vectors are independent.

etc.

Actually, I should add a corollary to that page: nullity is one iff column vectors are independent... --GFauxPas 23:32, 4 April 2012 (EDT)


 * Well, we do have Rank Plus Nullity Theorem, if that's what you mean. By the way, did you mean $\nu\left({\mathbf A}\right) = 0$? --abcxyz 23:47, 4 April 2012 (EDT)


 * Rank plus nullity is for transformations, the page doesn't yet tie it into matrices. Oh and actually I'm not sure what the dimension of $\left \{ {\mathbf 0} \right \}$ is, now that I think about it. --GFauxPas 00:17, 5 April 2012 (EDT)


 * I thought that an $m \times n$ matrix $\mathbf A$ with entries in $\R$ can be viewed as the linear transformation $\mathbf A : \R^n \to \R^m$ given by $\mathbf x \mapsto \mathbf A \mathbf x$. Isn't this already noted in Definition:Matrix?
 * I'm pretty sure that $\dim \left({\left\{{\mathbf 0}\right\}}\right) = 0$, since its basis is empty. --abcxyz 00:26, 5 April 2012 (EDT)


 * Yeah a matrix can be looked at as a LT but I'd need a theorem that the $n$ in $\rho + \nu = n$ is the number of columns. Anyway, the idea of an empty basis is kind of interesting. Though it fits the definition of empty sum of the linear combination of no vectors. Maybe. --GFauxPas 00:46, 5 April 2012 (EDT)


 * Well, if $\mathbf x \in \R^n$, then $\mathbf A$ must have $n$ columns for $\mathbf A \mathbf x$ to be defined ... right? So the domain of the linear transformation $\mathbf A : \R^n \to \R^m$ is $n$-dimensional; doesn't that permit the use of the rank-nullity theorem ($\rho + \nu = n$)? --abcxyz 01:00, 5 April 2012 (EDT)
 * It sure does - but it deserves a page. Thanks for the proof, shorter than I thought. --GFauxPas 01:21, 5 April 2012 (EDT)

Matrix Spaces
My linear algebra class gets to abstract vector spaces in around two weeks. In the mean time, doesn't hurt to try to get some understanding on my own. What's the proper notation and definition of a matrix such that each column is a member of a vector space? What's the proper way to say "the matrix $\mathbf A$ that when multiplied on the left on a vector does the same thing as some linear transformation $T$ on the vector"? --GFauxPas 09:05, 6 April 2012 (EDT)


 * 1. Same notation as block matrices, I think: $\left[{\begin{array}{cccc}\mathbf v_1 & \mathbf v_2 & \cdots & \mathbf v_n\end{array}}\right]$.
 * 2. Probably something like "matrix of the linear transformation $T$" or "transformation matrix of $T$". --abcxyz 11:21, 9 April 2012 (EDT)


 * Surely a reference needs to be made to the bases chosen. Something like 'the matrix of $T$ with respect to the bases $e_1\ldots e_n$ and $f_1\ldots f_m$'. --Lord_Farin 11:28, 9 April 2012 (EDT)

Matrix-Vector operations
--GFauxPas 11:03, 9 April 2012 (EDT)

Let $\mathbf A$ be an $m \times n$ matrix.

Let $T: \R^n \to \R^m, \mathbf x \mapsto \mathbf A \mathbf x$ be a mapping.

Then $T$ is a linear transformation.

From
 * $\forall \mathbf x, \mathbf y \in \R^n: \mathbf A \left({\mathbf x + \mathbf y}\right) = \mathbf A \mathbf x + \mathbf A \mathbf y$, from Matrix Multiplication Distributes over Matrix Addition.

Scalar product thing
What's the name of this property?

where $\mathbf A$ and $\mathbf B$ are matrices or vectors, $\lambda \in \R$. --GFauxPas 11:03, 9 April 2012 (EDT)


 * I believe it is generally called homogeneity (of degree 1). But usually one calls this and the additive property together, '$\R$-linearity'. --Lord_Farin 11:20, 9 April 2012 (EDT)


 * By additivity, you mean $\lambda\left({\mathbf A + \mathbf B}\right) = \lambda \mathbf A + \lambda \mathbf B$? And the PW page should be "Scalar Multiplication of Matricies is R-Linear", or something? --GFauxPas 11:28, 9 April 2012 (EDT)


 * By additivity I mean simply $T(v+w)=Tv+Tw$ (I'm too lazy to put mathbf). What you wrote is $\R$-linearity (or $\Bbb F$-linearity if $\lambda \in \Bbb F$). Oh and the plural of matrix is of course matrices (which you spelled phonetically). --Lord_Farin 11:31, 9 April 2012 (EDT)


 * Page names could be 'Matrix Multiplication is Additive' '.. is R-Linear' and '... is (R)-Homogeneous' (choose yourself if the R in the last name is desired). --Lord_Farin 11:33, 9 April 2012 (EDT)


 * Finally, what you wrote is that 'Scalar Multiplication of Matrices Distributes over Matrix Addition'. --Lord_Farin 11:34, 9 April 2012 (EDT)

Let $T: \R^n \to \R^m, \mathbf x \mapsto T\left({\mathbf x}\right)$ be a linear transformation.

Then $T\left({\mathbf x}\right) = \mathbf A_T \mathbf x$, where $\mathbf A_T$ is the $m \times n$ matrix:

Proof

As the codomain of $T$ is $R^m$, it follows that $\mathbf A_T$ has $m$ rows. Is that readily apparent, or do I need to explain it better? --GFauxPas 13:39, 11 April 2012 (EDT)


 * I would say it suffices to mention that each of the $T(e_i)$ has $m$ 'rows', if anything. --Lord_Farin 14:24, 11 April 2012 (EDT)