# Today I Learned

Some of the things I've learned every day since Oct 10, 2016

## 214: Orthogonal Matrices

In linear algebra, an orthogonal matrix is a square matrix $A$ such that

$AT^T = A^T A = I$.

Equivalently, the rows and columns of $A$ are, respectively, orthonormal vectors: vectors which are all unit length and orthogonal to one another.

Also equivalently, $A^{-1} = A^T$.

A fun fact about orthogonal matrices is that their determinants are always $\pm 1$.

## 81: General Linear Groups

The general linear group $\textrm{GL}(V)$, where $V$ is a vector space, is the group of all automorphisms $T: V \rightarrow V$ under the operation of composition of linear transformations.

When $V$ is over the field $F$ and is finite-dimensional with $\textrm{dim}(V) = n$, this group is isomorphic to the group of invertible $n \times n$ matrices with entries from $F$ under the operation of matrix multiplication. In this case, the group is often written as $\textrm{GL}(n, F)$.

General linear groups are used in group representations. A group representation is a representation of a group as a general linear group. That is, it is an automorphism

$f: G \rightarrow \textrm{GL}(V)$

where $G$ is the group being represented and $V$ is any vector space.

## 80: Multilinear Forms

In abstract algebra, a multilinear form is a mapping

$f: V^n \rightarrow F$

where $V$ is a vector space over the field $F$, such that each argument of $f$ is linear over $F$ with the other arguments held fixed. A special case of this is when $n = 2$ and $f$ is a bilinear form.

## 74: Linearly Separable Values (Euclidean)

2 sets $X_1, X_2$ of points in $n$-dimensional Euclidean space $E^n$ are linearly separable if and only if there exists a non-zero vector $\mathbf{w} \in E^n$ and a number $k$ such that

$\mathbf{w} \cdot \mathbf{x} < k$

holds for every $\mathbf{x} \in X_1$, and does not hold for every $\mathbf{x'} \in X_2$. Intuitively, this means that two sets of points in an $n$-dimensional Euclidean space are linearly separable if there is an $(n-1)$-dimensional plane that when inserted into the same space separates the two sets.

(This concept could probably be extended to spaces which share certain properties with $E^n$, such as having a partial order, closure, etc., but $E^n$ gives the simplest example.)

## 72: Bilinear Maps

In linear algebra, a bilinear map is a function $f: X \times Y \rightarrow Z$, where $X, Y, Z$ are vector spaces over a common field, which is linear in each of its 2 components when the other is held fixed. When $f(x, y) = f(y, x)$ for all $(x, y) \in X \times Y$, it is referred to as a symmetric bilinear map.

Examples of bilinear maps include matrix multiplication, the inner product, and the cross product.

## 57: Positive-Definite Matrices

A complex matrix $A$ is said to be positive-definite iff for every column vector $c \in \mathbb{C}^n$,  $c^{t} A c$ is real and positive. This is equivalent to the condition that all the eigenvalues of $A$ are positive.

Similarly, there are variations of positive-definiteness with analogous conditions:

$A$ positive-semidefinite $\leftrightarrow$ $c^{t} A c$ real and non-negative $\leftrightarrow$ non-negative eigenvalues

$A$ negative-semidefinite $\leftrightarrow$ $c^{t} A c$ real and non-positive $\leftrightarrow$ non-positive eigenvalues

$A$ negative-definite $\leftrightarrow$ $c^{t} A c$ real and negative $\leftrightarrow$ negative eigenvalues

## 56: Unitary Linear Operators

In linear algebra, a unitary operator $T$ over a inner product space $V$ is one which satisfies

$TT^* = T^*T = I$.

Thus it is a special kind of normal operator. The following conditions are equivalent to $T$ being unitary:

• $T$ preserves the inner product. That is, $\langle T(x), T(y) \rangle = \langle x, y \rangle$.
• $T$ is distance-preserving. That is, $||T(x)|| = ||x||$.
• $T^*$ is unitary.
• $T$ is invertible and $T^{-1} = T^*$.
• $T$ is a normal operator with eigenvalues on the complex unit circle.

and the following are additionally true of a unitary operator:

• $T$ is normal.
• The eigenspaces of $T$ are orthogonal.
• Every eigenvalue of $T$ has an absolute value of $1$.
• $U = P^*DP$ for some unitary transformations $D, P$, where $D$ is diagonal.

(When $T$ is over $\mathbb{R}$ it is sometimes referred to as ‘orthogonal’ rather than ‘unitary’.)

## 48: Self-Adjoint Linear Operators

A linear operator $T: V \rightarrow V$ is self-adjoint iff it is its own adjoint, i.e. iff

$\langle T(x), y \rangle = \langle x, T(y) \rangle \quad \forall x, y \in V$.

This is equivalent to the condition that the matrix of $T$ with respect to any orthonormal basis is Hermitian (the matrix is its own conjugate transpose).

In addition, if $T$ is self-adjoint, then there exists an orthonormal eigenbasis $\beta$ for $V$ such that the matrix representation of $T$ with respect to $\beta$ is a diagonal matrix with real entries.

## 47: Invariant Distributions as Eigenvectors

Since a stationary distribution $\pi$ of a finite Markov chain $X$ satisfies $\pi P = \pi$, where $P$ is the transition matrix of $X$, it can be seen as an eigenvector of eigenvalue $\lambda = 1$ under the linear transformation by $P$. Specifically, $\pi$ is the intersection of the eigenspace $E_1$ with the hyperplane formed by the constraint that $\sum _{i = 1} ^n \pi (i) = 1$.

(Here the vector space in question is $\mathbb{R}^n$, where $n$ is the number of states in $X$.)

## 42: Adjoint Operators

In linear algebra, if $T$ is a linear operator $T: V \rightarrow V$ over a finite vector space $V$, then there exists a unique $T^*: V \rightarrow V$, the adjoint operator, such that

$\langle T x, y \rangle = \langle x, T^* y \rangle$

for all $x, y \in V$.