# 0x011 Linear Algebra

Contents Index

## Vector Spaces

closed under vector addition and scalar multiplication. It is a specicial case of module: module over a field

### Vector Space

An addition on a set $V$ is a function that assigns $u+v \in V$ to each pair of elements $u,v \in V$

A scalar multiplication on a set $V$ is a function that assigns an element $\lambda v \in V$ to each $\lambda \in F$ and each $v \in V$

Definition (vector space)

A vector space is a set $V$ along with an addition on $V$ and a scalar multiplication on V such that the following properties hold:

• commutativity: $u+v = v+u$ for all $u,v \in V$
• associativity: $(u+v)+w = u + (v+w)$ and $(ab)v = a(bv)$ for all $u,v,w \in V$ and $a,b \in F$
• additive identity: there exists an element $0 \in V$ such that $v+0 = v$ for all $v \in V$
• additive inverse: for every $v \in V$, there exists $w \in V$ such that $v+w=0$
• multiplicative identity: $1v = v$ for all $v \in V$
• distributive properties: $a(u+v) = au + av$ and $(a+b)v = av + bv$ for all $a,b \in F$ and all $u,v \in V$

Note: $(V,+)$ is an abelian group

Definition (vector): elements of a vector space are called vectors or points

### Subspaces

Definition (subspace) A Subset $U$ of $V$ is called a subspace of $V$ if $U$ is also a vector space (using the same addition and scalar multiplication as on $V$)

Criterion (subspace) A subset $U$ of $V$ is a subspace iff $U$ satisfies the following three conditions

• additive identity: $0 \in U$
• closed under addition: $u,w \in \implies u+w \in U$
• closed under scalar multiplication: $a \in F, u \in U \implies au \in U$

Note: the union of subspaces is rarely a subspace, but a collection of subspaces are always subspace.

Definition (sum of subspace) Suppose $U_1, U_2, … U_m$ are subsets of $V$, the sum of $U_1, … U_m$, denoted by $U_1 + … + U_m$ is the set of all possible sums of elements of $U_1, … , U_m$. More precisely,

$$U_1 + U_2 + … + U_m = \{ u_1 + u_2 + … + u_m | u_1 \in U_1, …, u_m \in U_m \}$$

Definition (direct sum) Suppose $U_1, U_2, … U_m$ are subspaces of $V$, the sum $U_1 + U_2 + … + U_m$ is called a direct sum if each element of $U_1 + U_2 + … + U_m$ can be written in only one way as a sum of $u_1 + u_2 + … + u_m$, if it is a direct sum, then $U_1 \oplus U_2 \oplus … \oplus U_m$ denotes $U_1 + U_2 + … + U_m$

Criterion (direct sum) $U + W$ is a direct sum in either of the following case

• there is only one way to write $u+w = 0$, where $u \in U, w \in W$
• $U \cap W = \{ 0 \}$

note: direct sum means the linear independence between subspaces

### Finite Dimension Vector Space

Definition (linear combination) A linear combination of a list $v_1, …, v_m$ of vectors in $V$ is a vector of the form $a_1 v_1 + … + a_m v_m$ where $a_1, … , a_m \in F$

btw, Affine combination has a bias in addition

Definition (span) The set of all linear combinations of $v_1, …, v_m$ in $V$ is called the span of $v_1, … v_m$, denoted $span(v_1, …, v_m)$.

Definition (basis) A basis of $V$ is a list of vectors in $V$ that is linearly independent and spans $V$

$$v = a_1 v_1 + … + a_n v_n$$

Note that it has to satisfy two properties: 1) linear independence 2) span the whole space.

## Inner Product Space

### Inner Products and Norms

inner product is a generalization of the dot product

Definition (inner product) An inner product on $V$ is a function that takes each ordered pair $(u, v)$ of elements of $V$ to a number $\langle u, v \rangle \in F$ and has the following properties.

• (positivity) $\langle v, v \rangle \geq 0$ for all $v \in V$
• (definiteness) $\langle v, v \rangle = 0$ iff $v = 0$
• (additivity in first slot) $\langle u+v, w \rangle = \langle u, w \rangle + \langle v, w \rangle$ for all $u,v,w \in V$
• (homogeneity in first slot) $\langle \lambda u, v \rangle = \lambda \langle u, v \rangle$ for all $\lambda \in F$ and all $u,v \in V$
• (conjugate symmetry) $\langle u, v \rangle = \overline{\langle v, u \rangle}$ for all $u,v \in V$

Definition (inner product space) An inner product space is a vector space $V$ along with an inner product on $V$

Definition (norm) For $v \in V$, the norm of $v$, denoted $||v||$ is defined by

$$||v|| = \sqrt{\langle v, v \rangle}$$

Definition (orthogonal) Two vectors $u, v \in V$ are called orthogonal if $\langle u, v \rangle = 0$

Theorem (Pythagorean) Suppose $u, v$ are orthogonal in $V$. Then

$$|| u+v ||^2 = ||u||^2 + ||v||^2$$

Theorem (Cauchy-Schwarz) Suppose $u,v \in V$. Then

$$| \langle u, v \rangle | \leq ||u|| ||v||$$

Theorem (Triangle Inequality) Suppose $u, v \in V$. Then

$$||u+v|| \leq || u|| + ||v||$$

### Orthonormal Bases

Definition (orthonormal) A list of vectors is called orthonormal if each vector in the list has norm 1 and isorthogonal to all the other vectors in the list.

Definition (orthonormal basis) An orthonormal basis of $V$ is an orthonormal list of vectors in $V$ that is also a basis of $V$

Theorem (Schur) Suppose $V$ is a finite-dimensional complex vector space and $T \in \mathcal{L}(V)$ Then $T$ has an upper-triangular matrix with respect to some orthonormal basis of $V$

Theorem (Riesz) Suppose $V$ is finite-dimensional and $\varphi$ is a linear functional on $V$. Then there is a unique vector $u \in V$ such that

$$\varphi(v) = \langle v, u \rangle$$

### Orthogonal Complements

Definition (orthogonal complement) If $U \subset V$, then the orthogonal complement of $U$, denoted $U^\perp$ is

$$U^\perp = \{ v \in V : (\forall u \in U) \langle v, u \rangle = 0 \}$$

Lemma (direct sum decomposition with orthogonal complement) Suppose $U$ is a finite-dimensional subspace of $V$. Then

$$V = U \oplus U^\perp$$

## Linear Map

### vector space of linear maps

Definition (linear map) A linear map from vector space $V$ to vector space $W$ obeys following properties

• T preserve addition (additivity) : $T(x+y)=T(x)+T(y)$
• T preserves scalar multiplication (homogeneity): $T(cx) = cT(x)$

Linear map is homomorphism of vector space

Definition (add,mul in linear maps) The set of all linear maps from $V$ to $W$ is denoted $\mathcal{L}(V, W)$. Suppose $S,T \in \mathcal{L}(V,W), \lambda \in F$ then sum and product are linear maps from $V \to W$

$$(\forall v \in V) (S+T)(v) = Sv + Tv$$

$$(\forall v \in V) (\lambda T)(v) = \lambda(Tv)$$

With previous definitions, $\mathcal{L}(V,W)$ forms a vector space

Definition (production of linear maps) Suppose $T \in \mathcal{L}(U,V), S \in \mathcal{L}(V,W)$ then the product $S,T \in \mathcal{L}(U,W)$ is defined by

$$(\forall u \in U) (ST)(u) = S(T(u))$$

Proposition (properties of products)

• associativity $(T_1 T_2)T_3 = T_1 (T_2 T_3)$
• identity $TI = IT = T$
• distributive properties $(S_1 + S_2)T = S_1 T + S_2 T$

### Injectivity and Surjectivity

Both injectivity and subjectivity are important concepts in linear algebra, they can be characterized by the null space and range space.

Definition (injective) A function $T: V \to W$ is called injective if $Tu = Tv \implies u=v$

Defintiion (null space) Null space (kernel) of linear map $T \in \mathcal{L}(V,W)$ is a subset of $V$ which was mapped to 0 vector :

$$null(T) = \{ v \in V: T(V) = 0 \}$$

Note that null space is a subspace of $V$

Injectivity and null space are connected by the following proposition

Condition (injectivity) Let $T \in \mathcal{L}(V,W)$, then $T$ is injective iff $null(T)=\{ 0 \}$

Surjectivity is the property whether all elements in the target space can be reached with the map

Definition (surjective) A function $T: V \to W$ is called surjective if its range is $W$

Definition (range) The range of $T: V \to W$ is the subset of $W$ consisting of those vectors that are of the form $Tv$ for some $v \in V$

$$range(T) = \{ Tv : v \in V \}$$

Note that range is a subspace of $W$

Surjectivity can be related to range space by the following condition.

Condition (surjective) A function $T: V \to W$ is called surjective if its range is $W$

Surjectivity/Injectivity or range/null can be related by the following fundamental theorem in Linear algebra.

Theorem (fundamental theorem of linear maps) Suppose $V$ is finite-dim and $T \in \mathcal{L}(V,W)$ .then range $T$ is finte-dim and

$$\dim V = \dim null T + \dim range T$$

Proof concept: suppose $u_1, …, u_m$ is a basis of $null(T)$, extending it to the basis of $V$ by adding $v_1, …, v_n$, then showing that $Tv_1, …, Tv_n$ is a basis of $range(T)$

### Invertibility (isomorphism)

Definition (invertible) A linear map $T \in \mathcal{L}(V,W)$ is called invertible if there exists a linear map $S \in \mathcal{L}(W,V)$ such that $ST$ and $TS$ are identity maps. $S$ and $T$ are called an inverse of the other respectively.

The inverse of $T$ is denoted by $T^{-1}$, and it is unique.

Proposition (invertibility == injectivity + surjectivity) A linear map is invertible iff it is injective and surjective

Definition (isomorphism, isomorphic) An isomorphism is an invertible linear map, two vector spaces are called isomorphic if there is an isomorphism from one onto the other

Proposition (dimension and isomorphism) Two finite-dim vector spaces are isomorphic iff they have the same dim

Definition (operator) An operator $T \in \mathcal{L}(V)$ is Da linear transformation $T$ for a vector space $V$ to itself. (endomorphism)

Criterion (invertibility) Following statements are equivalent in operator $T$ in finite dimensional vector space

• $T$ is invertible
• $T$ is injective
• $T$ is surjective

Note that neither injectivity nor surjectivity implies invertibility in infinite-dim vector spaces

### Products

Definition (product of vector spaces) The product of vector space is defined by

$$V_1 \times V_2 … \times V_m = \{ (v_1, …, v_m) : v_1 \in V_1, …, v_m \in V_m \}$$

Definition (sum of vector and subspace) suppose $v \in V$ and $U$ is a subspace of $V$.

$$v + U := \{ v + u : u \in U \}$$

Defintion (affine subset, parallel) An affine subset of $V$ is a subset of $V$ of the form $v + U$ where $v \in V$ and $U$ is a subspace of $V$, and the affine subset $v+U$ is said to be parallel to $U$

### Quotients

following binary relation on $V$ where $S \subset V$ is a subspace is an equivalence relation

$$u \equiv v \iff u – v \in S$$

Definition (quotient space) Suppose $U$ is a subspace of $V$, then the quotient space $V/U$ is the set of all affine subsets of $V$ parallel to $U$

$$V/U := \{ v + U : v \in V \}$$

The set $v+U$ is called a coset of $U$ and $v$ is called a coset representative.

Definition (addition, scalar multiplication on quotient space)

$$(v+U) + (w+U) = (v+w) + U$$

$$\lambda (v+U) = (\lambda v) + U$$

quotient space forms vector space with these definitions. However, not all structures in algebra has this property, for example subgroup. Only normal subgroup has this property where $G/N$ is a group. For rings, ideals have this property.

Definition (quotient map, canonical projection) quotient map (actually a linear map) $\pi: V \to V/U$ is defined

$$\pi(v) = v + U$$

Definition ($\widetilde{T}$) Suppose $T \in \mathcal{L}(V,W)$. Define $\widetilde{T}: V/(null T) \to W$ by

$$\widetilde{T}(v + null T) = Tv$$

Theorem (first isomorphism theorem) $\widetilde{T}$ is injective

### Duality

Definition (linear functional) A linear functional on $V$ is a linear map from $V$ to $F$. In other words, it is an element of $\mathcal{L}(V, F)$

Definition (dual space) The dual space of $V$, denoted $V’=\mathcal{L}(V, F)$, is the vector space of all linear functionals on $V$

Definition (dual basis) dual basis of $V$’s basis $v_1, …, v_n$ is the list $\varphi_1, … \varphi_n$ of element of $V’$ where $\varphi_j(v_k)=1$ iff $k==j$, otherwise 0. Dual basis is a basis of $V’$

Definition (dual map) dual map of $T \in \mathcal{L}(V,W)$ is the linear map $T’ \in \mathcal{L}(W’, V’)$ defined by $T'(\varphi)=\varphi \circ T$

Definition (annihilator) The annihilator of $U \subset V$, denoted by $U^{\circ}$ is the subspace of linear functionals collapsing $U$ to 0

$$U^{\circ} = \{ \varphi \in V’ | (\forall u \in U) \varphi(u) = 0 \}$$

## Eigenvalues and Eigenvectors

### Invariant Subspaces

Definition (invariant subspace) Suppose $T \in \mathcal{L}(V)$ A subspace $U$ of $V$ is called invariant under $T$ if $u\in U \implies Tu \in U$

$U$ is invariant under $T$ is $T|_{U}$ is an operator on $U$

Note: ${0}, V, null(T), range(T)$ are invariant subspaces of $T$.

Definition (eigenvalue, eigenvector) Suppose $T \in \mathcal{L}(V)$. A number $\lambda \in F$ is called an eigenvalue of $T$ if there exists a non-zero $v \in V$ and $Tv = \lambda v$, and $v$ is called eigenvector of $T$ corresponding to eigenvalue $\lambda$

note: eigenvector is the basis of 1-dim invariant subspace of $T$

Criterion (eigenvalue) Suppose $V$ is finite-dimensional, $T \in \mathcal{L}(V), \lambda \in F$. Then $lambda$ is an eigenvalue of $T$ in either of the following case

• $T – \lambda I$ is not injective
• $T – \lambda I$ is not surjective
• $T – \lambda I$ is not invertible

Definition (restriction and quotient operators) Suppose $T \in \mathcal{L}(V)$ and $U$ is a subspace of $V$ invariant under $T$

The restriction operator $T|_{U} \in \mathcal{L}(U)$ is defined by

$$\forall (u \in U) T|_{U}(u) = Tu$$

The quotient operator $T/U \in \mathcal{L}(V/U)$ is defined by

$$\forall (u \in U) (T/U)(v+U)=Tv + U$$

### Upper-Triangular Matrices

The main reason that a richer theory exists for operators than for more general limear maps is that operators can be raised to powers

Definition (matrix of an operator) Suppose $T \in \mathcal{L}(V)$ and $v_1, …, v_n$ is a basis of $V$. The matrix of $T$ with respect to this basis is the n-by-n matrix whose entries $A_{j,k}$ are defined by $Tv_k = A_{1,k} v_1 + … + A_{n,k} v_n$

Definition (diagonal of a matrix) The diagonal of a square matrix consists of the entries along the line from the upper left corner to the bottom right corner

Definition (upper-triangular matrix) A matrix is called upper triangular if all the entries below the diagnoal equal 0

Proposition (conditions for upper-triangular matrix) Suppose $T \in \mathcal{L}(V)$ and $v_1, …, v_n$ is a basis of $V$. Then the following are equivalent

• the matrix of $T$ with respect to $v_1, …, v_n$ is upper triangular
• $(\forall j) Tv_j \in span(v_1, …, v_j)$
• $span(v_1, …, v_j)$ is invariant under $T$ for each $j$

## Reference

 Axler, Sheldon Jay. Linear algebra done right. Vol. 2. New York: Springer, 1997.

 Roman, Steven, S. Axler, and F. W. Gehring. Advanced linear algebra. Vol. 3. New York: Springer, 2005.

 Boyd, Stephen, and Lieven Vandenberghe. Introduction to applied linear algebra: vectors, matrices, and least squares. Cambridge university press, 2018.