Note that Vectors need not be . Any possible element of a vector space is by definition, a vector. This means that vector spaces really contain Tensors. 99% of the time when people are talking about vectors, they really mean “Coordinate vector”, i.e. a vector that represents a location in Space. Whereas a vector represents a location in a Vector Space.
An element of a Vector Space, represented in that space either by a direction and magnitude, or by a list of Numbers, each describing the placement along a coordinate axis.
While, a subspace does inherit the axioms of a Vector Space. Verifying a subset is a subspace is simpler; A Subset of is a subspace if it is closed within scalar multiplication and vector addition:
Properties
, i.e. the Span of any subset is identical to itself
This is because a basis requires a Set of Linearly IndepedentVectors, and does not meet this requirement. Therefore, its only basis is . The cardinality of the empty set is 0, therefore so is
Examples
dim
dim(Nul A) is the number of
number of free vars of A
dim(Col A) is the number of
number of pivots of A
H =
n variables, solve for ito everything else. → one pivot everything else free vars. Therefore free vars
Imagine the area of parallelogram created by the basis of a standard vector space, like . Now apply a linear transformation to that vector space. The new area of the new parallelogram has been scaled by a factor of the determinant.
is the parallelopiped.
You can also just think of it as the area of the parallelogram spanned by the columns of a matrix
: parallelopiped and volume
(assume n by n matrix because we only know how to find determinants for square matrices)
You can also get the area of S by using the determinant of the matrix created by the vectors that span S, i.e.
because you are shifting the standard basis vectors into the vector space dictated by S
Determinant Laws
det(A) = 0 A is singular
det(A) 0 A is invertible
det(Triangular) = product of diagonals
det A = det
det(AB) = det A · det B
Determinant Post Row Operations
if A square:
if adding rows to rows on A to get B then
if swapping rows in A to get B then
if scaling one row of A by k, then =
Exactly the same for columns
Cofactor expansion is a method used to calculate the Determinant of an matrix . It works by reducing the determinant of the matrix to a sum of determinants of submatrices. It is a recursive definition.
where is the determinant of the submatrix of obtained by deleting the -th row and -th column
The signs for each term of the expansion are determined solely by the position and follow a checkerboard pattern, starting with in the top-left corner
The determinant of a matrix can be calculated by cofactor expansion along any single row or any single column .
Expansion Along Row :
Expansion Along Column :
Strategy: To simplify calculations, it is generally best to choose the row or column with the most zeros.
is an eigenvector for
An eigenvector is a vector solution to the above equation, such that the linear transformation of has the same result as scaling the vector by .
Eigenvalue
is the corresponding eigenvalue ()
Solve for in , which yields the Characteristic Equation for this system, e.g. in a 2D systems it is . In a 3D+ system, you still have to create the characteristic equation but it requires
Notes:
point same direction
point opposite direction
can be complex even if nothing else in the equation is
Eigenvalues cannot be determined from the reduced version of a matrix ⭐
i.e. row reductions change the eigenvalues of a matrix
The diagonal elements of a triangular matrix are its eigenvalues.
A invertible iff 0 is not an eigenvalue of A.
Stochastic matrices have an eigenvalue equal to 1.
If are eigenvectors that correspond to distinct eigenvalues, then are linearly independent
Defective
An eigenvalue is defective if and only if it does not have a complete Set of Linearly Independent eigenvectors.
Due to ‘s contribution,
Neutral Eigenvalue
Eigenspace
the span of the eigenvectors that correspond to a particular eigenvalue
to get values for . Recall means noninvertible. If a matrix isn’t invertible, then we won’t get trivial solutions when solving. Also the idea of reducing the dimension through the transformation is relevant; squishing the basis vectors all onto the same span where the area/volume is 0. Recall eigenvectors remain on the same span despite a linear transformation.
is an eigenvalue of A is singular
trace of a Matrix is the sum of diagonal
Characteristic Polynomial
n degree polynomial → n roots → maximum n eigenvalues (could be repeated)
Algebraic Multiplicity
Algebraic multiplicity of an eigenvalue is how many times an eigenvalue repeatedly occurs as the root of the characteristic polynomial.
Geometric Multiplicity
Geometric multiplicity of an eigenvalue is the number of eigenvectors associated with an eigenvalue; , which is saying how many eigenvector solutions does this eigenvalue have (recall is number of free vars in )
An Equation where in the above form where the the a’s are coefficients, x’s are variables, n is the dimension (number of variables), e.g. has two dimensions
A linear system is considered consistent if it has solution(s)
Theorem for Consistency
A linear system is consistent iff the last
column of the augmented matrix does not have a pivot. This is
the same as saying that the RREF of the augmented matrix does
not have a row of the form
Moreover, if a linear system is consistent, then it has
1. a unique solution iff there are no free variables.2. infinitely many solutions that are parameterized by free variables
Row Equivalent
Row Equivalent
If two matrices are row equivalent they have the same solution set, meaning that they have a Bijection through Row Operations
Homogenous systems always have a trivial solution, so naturally we would like to know if there are nontrivial, perhaps infinite solutions, namely if there is a free variable and a column with no pivots
The only solution to is the trivial Linearly Independent
Geometric Interpretation
If two vectors are linearly independent, the are not colinear
If 3, then not coplanal
If 4, not cospacial
Linear Dependence
Any of the vectors in the set are a linear combination of the others
If there is a free variable, so there are infinite solutions to the homogenous equation
If the columns of A are in , and there are basis vectors in (which is always true), then if the amount of columns in A exceeds the amount of basis vectors that exist in that dimension, it means that there are free variables, which indicates linear dependence
A standard matrix is a Matrix that represents a Linear Transformation, where the transformation can be expressed as . It is found by applying the transformation to the Standard Basis Vectorss and using the results as the columns of the matrix
Theorem
Let be a Linear Transformation. Then there is a unique Matrix such that
In fact, is a and its column is a vector
When doing multiplication with a block matrix, make sure the “receiving” matrix’s entries go first, to respect the lack of commutativity in matrix multiplication. See Multiplication
Suppose A can be row reduced to echelon form U without interchanging rows, i.e.
You can construct by finding each and multiplying them all together. Alternatively you can construct such that the sequence of row operations that convert to would convert to
is diagonalizable has linearly independent eigenvectors.
i.e.
where vectors are linearly independent eigenvectors, and are the corresponding eigenvalues, in order.
Distinct Eigenvalues
If and has distinct eigenvalues, then is diagonalizable
Non-distinct Eigenvalues
You check that the sum of the geometric multiplicities is equal to the size of the matrix.
e.g. for
Find the eigenvalues:
We know that geomult ⇐ algmult. Therefore has 1 distinct eigenvector.
This means has to have 2 distinct eigenvectors to form a basis, so if it doesn’t then the matrix is not diagonalizable.
There is only one free columns here. Therefore, the dimension of the Nullspace is one, not two, which means the matrix is not diagonalizable.
Complex roots coming in complex conjugate pairs implies that complex Eigenvalues and their Eigenvectors come in complex conjugate pairs. This is because eigenvalues are the solutions to the Characteristic Equation, which is a Polynomial equation.
I proceed to define the orthogonal decomposition for some vector , where , where is a subspace of , where is the Orthogonal Complement of , where is the orthogonal projection of onto , and is a vector orthogonal to . This is a Vector Decomposition.
Every has a unique sum in the form above, so long as is a subspace of
Concerning
If is an Orthogonal Basis for , then , the orthogonal projection of onto is given by:
Let be a set of vectors that form a basis for subspace of ,
Let be a set of vectors that form an Orthogonal Basis for
The Gram-Schmidt process defines how can be derived from
It depends on Orthogonal Decomposition
Given many data points, construct a matrix equation in the form of a linear equation (this matrix equation will be overdetermined). The matrix equation below is
This equation is linear but it doesn’t have to be, just adjust accordingly to represent the equations as a matrix equation.
is a subspace of , , i.e.
Note: Can only use Orthogonal Decomposition for when the columns of A form an orthogonal basis, by definition
In other words, is closest vector in to
Note: is a unique vector, a special that minimizes the above equation.
Note: If the columns of are orthogonal, then you can just use the scalar projection of onto each column of .
Normal Equations
The least squares solutions to corresponds to the solution to
Turns the equation from above and transforms it into a square matrix equation
Derivation
is the Least Squares Solution to
Normal Equation Usage
Use when non-square matrix
Over/Under determined
Regression
Theorem (Unique Solutions for Least Squares)
If A is m x n
Ax = b has a unique least squares solution for each b in Rm
Cols of A are linearly independent
The matrix A^T A is invertible
If the above hold, the unique least square solution is
If the above conditions are not true, there may be infinitely many solutions, or some other nonunique amount of solutions, in which case you should consider instead.
Note: plays the role of the “length squared” of the matrix A
Theorem (Least Squares and QR)
Examples
Hampton Explanation for Least Squares
Let . is the unique, minimizing solution to the equation such that
Essentially, minimize
is the minimal distance between the different solutions
Goal: Find s.t. is closest to
in this context just denotes the special/unique that minimizes the distances between and
b is closer to Axhat than to Ax for all other x in Col A
If b in Col A, then xhat is…
Seek so that is as close to as possible, i.e. should solve Axhat = bhat