# Linear Algebra

This is a matrix.

$\begin{bmatrix} 1 & 2 & 3 \\\\ 4 & 5 & 6 \end{bmatrix}$

It has 2 rows and 3 columns, so it is a $$2 \times 3$$ matrix.

Matrix addition and multiplication-by-a-scalar is done componentwise. Matrix multiplication is done trickily; it’s associative, distributive, commutative with scalars, linear, anticommutative-under-transposition.

Vectors are like columns of matrices, or matrices with one column. Except row vectors are rows of matrices.

The three elementary row operations are:

1. exchanging two rows
2. multiplying a row by a nonzero scalar
3. adding a multiple of a row to another row

They can each be represented by multiplying by a simple matrix to the left. Also, they’re uniquely reversible.

A matrix is in echelon form if the nonzero elements are like an inverted staircase. This is vague because echelon form is not unique and usually you can just row-reduce as well. A matrix is in row-reduced echelon form if the nonzero elements are like an inverted staircase and every column where the staircase steps down has one 1 and the rest 0s. Row-reduced echelon form is useful for proving stuff. In both cases, the first nonzero entry in a nonzero row is called a pivot entry and the column it belongs to is called a pivot column. Other columns are nonpivot or free.

A set of vectors, or anything else that behaves kind of like vectors, is a vector space if it’s closed under addition and multiplication by scalars. Also, it should probably be nonempty, so it includes $$\vec{\textbf{0}}$$.

The nullspace of a matrix $$A$$ is the space of column vectors $$\vec{\textbf{X}}$$ so that $$AX = \vec{\textbf{0}}$$.

Vector spaces have unique dimension, i.e. all spanning bases have the same size. Proof: Take one spanning base and express it in terms of another. Adjust the base elements one by one to match each other.

The dimension of the nullspace is the nullity.

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