MA106 - Linear Algebra Flashcards

1
Q

Define what is meant by a Matrix

A

An m × n matrix A over R is an m × n rectangular array of real numbers. The entry in row i and column j is often written aij. We write A = (aij) to make things clear.

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2
Q

Define Addition of Matrices

A

Let A = (aij ) and B = (bij ) be two m × n matrices over R. We define A+B to be the m×n matrix C = (cij ), where cij = aij +bij for all i, j.

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3
Q

Define Scalar Multiplication of Matrices

A

Let A = (aij) be an m × n matrix over R and let β ∈ R. We define the scalar multiple βA to be the m×n matrix C = (cij), where cij = βaij for all i,j.

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4
Q

Define Multiplication of Matrices

A
Let A = (aij ) be an l × m matrix over R and let B = (bij) be an m × n matrix over K. The product AB is an l × n matrix C = (cij) where, for 1 ≤ i ≤ l and 1 ≤ j ≤ n,
cij =sum(k=1 to m) 􏰌aikbkj =ai1b1j +ai2b2j +···+aimbmj.
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5
Q

Define what is meant by the Zero Matrix

A

The m × n zero matrix 0mn has all of its entries equal to 0.

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6
Q

Define the Identity Matrix

A

The n × n identity matrix In = (aij ) has aii = 1 for 1 ≤ i ≤ n, but aij =0wheni̸=j.

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7
Q

What are the Properties for a Matrix to be in Upper Echelon Form?

A

(i) All zero rows are below all non-zero rows.

(ii) Let r1,…,rs be the non-zero rows. Then each ri with 1 ≤ i ≤ s has 1 as its
first non-zero entry. In other words, ai,c(i) = 1 for all i ≤ s.

(iii) The first non-zero entry of each row is strictly to the right of the first non-zero
entry of the row above: that is, c(1) < c(2) < ··· < c(s).

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8
Q

What are the Properties for a Matrix to be in Row Reduced Form?

A

(i) All zero rows are below all non-zero rows.

(ii) Let r1,…,rs be the non-zero rows. Then each ri with 1 ≤ i ≤ s has 1 as its
first non-zero entry. In other words, ai,c(i) = 1 for all i ≤ s.

(iii) The first non-zero entry of each row is strictly to the right of the first non-zero
entry of the row above: that is, c(1) < c(2) < ··· < c(s).

(iv) If row i is non-zero, then all entries both above and below the first non-zero entry of row i are zero: ak,c(i) = 0 for all k ̸= i.

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9
Q

Define the types of Elementary Row Operations.

A

(R1) For some i ̸= j, add a multiple of rj to ri.

(R2) Interchange two rows.

(R3) Multiply a row by a non-zero scalar

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10
Q

Define what is meant by an Augmented Matrix.

A

For the system Ax = b of m equations in n unknowns.
If A is an m x n matrix, b=(bi1) The augmented matrix is the m x (n+1) matrix with
(ai1, ai2, …, ain | bi)

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11
Q

Define the types of Elementary Column Operations

A

(C1) For some i ̸= j, add a multiple of cj to ci.

(C2) Interchange two columns.

(C3) Multiply a column by a non-zero scalar.

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12
Q

What does a Matrix look like in Row and Column Reduced Form?

A

It is the Identity Is, surrounded by 0s from m-s to s or n-s to s

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13
Q

What is another term for Row and Column Reduced Form?

A

Smith Normal Form

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14
Q

Define what is meant by a Field.

A

A field is the data of a set S, two special elements 0 ̸= 1 ∈ S, and two maps S × S → S, called addition and multiplication, respectively satisfying certain axioms. We write α + β for the result of applying the addition map (α, β), and αβ for the result of applying the multiplication map to (α,β).

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15
Q

What are the Axioms for Addition?

A

A1. α+β=β+α Ɐ α,β∈S.

A2. (α+β)+γ=α+(β+γ) Ɐ α,β,γ∈S.

A3. ∃ 0∈S st. α+0=0+α=α Ɐ α∈S.

A4. Ɐ α ∈ S ∃ −α ∈ S s.t. α+(−α) = (−α)+α = 0.

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16
Q

What are the Axioms for Multiplication?

A

M1. α.β=β.α Ɐ α,β∈S.

M2. (α.β).γ = α.(β.γ) Ɐ α, β, γ ∈ S.

M3. ∃ 1∈S s.t. α.1=1.α=α Ɐ α∈S.

M4. Ɐ α ∈ S with α ̸= 0, ∃ α^−1 ∈ S s.t.
α.α^−1 = α^−1.α = 1.

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17
Q

What is the Axiom relating Addition & Multiplication?

A

D. (α+β).γ=α.γ+β.γ Ɐ α,β,γ∈S.

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18
Q

What are the Axioms for Addition, Multiplication and Division?

A

A1. α+β=β+α Ɐ α,β∈S.

A2. (α+β)+γ=α+(β+γ) Ɐ α,β,γ∈S.

A3. ∃ 0∈S st. α+0=0+α=α Ɐ α∈S.

A4. Ɐ α ∈ S ∃ −α ∈ S s.t. α+(−α) = (−α)+α = 0.

M1. α.β=β.α Ɐ α,β∈S.

M2. (α.β).γ = α.(β.γ) Ɐ α, β, γ ∈ S.

M3. ∃ 1∈S s.t. α.1=1.α=α Ɐ α∈S.

M4. Ɐ α ∈ S with α ̸= 0, ∃ α^−1 ∈ S s.t.
α.α^−1 = α^−1.α = 1.

D. (α+β).γ=α.γ+β.γ Ɐ α,β,γ∈S.

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19
Q

Define what is meant by a Vector Space

A

A vector space over a field K is a set V with 2 basic operations, Addition and Scalar Multiplication, satisfying certain requirements. Thus Ɐ u,v∈V, u+v∈V is defined ,and Ɐ α∈K, αv∈V is defined. For V to be called a vector space, the following axioms must be satisfied Ɐ α, β ∈ K and Ɐ u, v ∈ V:

(i) Vector addition satisfies axioms A1-4
(ii) α(u+v)=αu+αv;
(iii) (α+β)v=αv+βv;
(iv) (αβ)v = α(βv)
(v) 1v = v.

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20
Q

Define what is meant by a Linear combination

A

A linear combination of v1, v2, . . . , vn is a vector of the form:
α1v1 + α2v2 + · · · + αnvn
for α1, α2, . . . , αn ∈ K.

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21
Q

What is meant by the Trivial Linear Combination?

A

0v1 + · · · + 0vn = 0

22
Q

Define Linear Independence

A

Let V be a vector space over the field K. The vectors v1, . . . , vn ∈ V are linearly dependent if there is a non-trivial linear combination of v1, . . . , vn that equals the zero vector.
The vectors v1,…,vn are linearly independent if they are not linearly dependent.
A set S ⊂ V is linearly independent if every finite subset of S is linearly independent.

23
Q

Define what is meant by Spanning Vectors

A

A subset S ⊂ V spans V if every v ∈ V is a finite linear combination:
α1v1 + α2v2 +···+ αnvn
with v1,…,vn ∈S.

24
Q

Define what is meant by a Basis of a Vector Space

A

A subset S ⊂ V is a basis of V if S is linearly independent, and spans V . If the basis S is finite we also choose an ordering of its vectors, and write S = {v1,…,vn}.

25
Define the Coordinates of V wrt the basis: | v1, v2, ..., vn
If v = α1v1 + · · · + αnvn | The scalars α1, . . . , αn are the coordinates of v wrt the basis v1, . . . , vn.
26
Define the Dimension of a Vector Space.
The number n of vectors in a basis of the finite-dimensional vector space V is called the dimension of V and we write dim(V ) = n.
27
Define what is meant by a Subspace.
A subspace of V is a non-empty subset W ⊆ V such that (i) W isclosedunderaddition: u,v∈W ⇒u+v∈W; (ii) W is closed under scalar multiplication: v ∈ W, α ∈ K ⇒ αv ∈ W .
28
Define W1 + W2 where W1, W2 are subspaces of V
W1 + W2 is the set of vectors v∈V s.t. v=w1+w2 for some w1 ∈ W1, w2 ∈ W2. W1+W2 ={w1 + w2 |w1 ∈ W1,w2 ∈ W2}.
29
Define what is meant by a Linear Transformation.
``` Let U,V be two vector spaces over the field K. A linear map T from U to V is a function T : U → V s.t.: (i) T(u1+u2)=T(u1)+T(u2) Ɐ u1,u2 ∈U; (ii) T(αu)=αT(u) Ɐ α∈K , u∈U. ```
30
Define the Identity Map.
The Identity Map IV : V → V by: | IV (v) = v Ɐ v∈V.
31
Define the Zero Map.
The Zero Map 0U,V : U → V by: | 0U,V(u)=0V Ɐ u∈U.
32
Define Addition of Linear Maps
T1 + T2 : U → V by the rule | (T1 + T2)(u) = T1(u) + T2(u) Ɐ u ∈ U.
33
Define Scalar Multiplication of Linear Maps
αT1 : U → V by the rule | (αT1)(u) = αT1(u) Ɐ u ∈ U.
34
Define Composition of Linear Maps
T2T1 : U → W by | (T2T1)(u) = T2(T1(u)) Ɐ u ∈ U.
35
Define what is meant by the Image of a Linear Map
The image of T, denoted im(T) is the set of v∈V s.t. v=T(u) for some u∈U.
36
Define What is Meant by the Kernel of a Linear Map
The kernel of T , denoted ker(T) is the set of u ∈ U s.t. T(u) = 0V .
37
Define Rank and Nullity of a Linear Map
(i) dim(im(T)) is called the rank of T; | (ii) dim(ker(T)) is called the nullity of T.
38
Define Row Space and Row Rank of a matrix A.
The row space of A is the subspace of K^n spanned by the rows r1,...,rm of A. The row rank of A is equal to the dimension of the row space of A. Equivalently, the row rank of A is equal to the size of the largest linearly independent subset of r1, . . . , rm.
39
Define what it means for a Linear Map/Matrix to be Invertible
Let T : U → V be a linear map with corresponding m × n matrix A. If ∃ T^−1:V →U s.t. TT^−1 =IV andT^−1T =IU Then T is Invertible, T^-1 is the Inverse A^−1 is the (n×m) matrix of T^−1, then: AA^−1 = Im and A^−1A = In. A is Invertible with Inverse A^-1
40
Define the Determinant of a Matrix
det(A) = 􏰌Sum(over φ∈Sn): | sign(φ)a1φ(1).a2φ(2) . . . anφ(n).
41
Define what is meant for a Matrix to be Upper Triangular
A matrix is called upper triangular if all of its entries below the main diagonal are zero; (aij) is upper triangular if aij = 0 Ɐ i > j.
42
Define what is meant for a Matrix to be Diagonal
A matrix is diagonal if all entries not on the main diagonal are zero; aij =0 Ɐ i̸=j.
43
Define the Transpose of a Matrix
The Transpose A^T of A is the n×m matrix (bij): | where bij =aji for1≤i≤n, 1≤j≤m.
44
Define the 3 Elementary Matrices
``` E(n)1λ,i,j (i ̸= j) is the an n × n matrix equal to the identity, but with an additional non-zero entry λ in the (i,j) position. Corresponds to (R1) ``` ``` E(n)2i,j is the n × n identity matrix with its ith and jth rows interchanged. Corresponds to (R2) ``` ``` E(n)3λ,i (where λ ̸= 0) is the n × n identity matrix with its (i, i) entry replaced by λ. Corresponds to (R3) ```
45
Define a Minor of a Matrix
Let A = (aij) be an n×n matrix. Let Aij be the (n−1)×(n−1) matrix obtained from A by deleting the ith row and the jth column of A. Now let Mij = det(Aij). Then Mij is called the (i,j)th minor of A.
46
Define a Cofactor of a Matrix
We define cij to be equal to Mij if i + j is even, and to −Mij if i + j is odd. Or, more concisely, cij = (−1)i+jMij = (−1)i+j det(Aij). Then cij is called the (i,j)th cofactor of A.
47
Define what is meant by the Adjugate Matrix
Let A be an n × n matrix. We define the adjugate matrix adj(A) of A to be the n × n matrix of which the (i, j)th element is the cofactor cji. In other words, it is the transpose of the matrix of cofactors.
48
Define Similarity of Matrices
Two n × n matrices, A,B over K are said to be similar if there exists an n × n invertible matrix P with B = P−1AP.
49
Define what is meant by Diagonisable
A matrix which is similar to a diagonal matrix is said to be diagonalisable.
50
Give the definition of Eigenvectors/Eigenvalues of a Linear Map
Let T : V → V be a linear map, where V is a vector space over K. Suppose that for some non-zero vector v ∈ V and some scalar λ ∈ K, we have T (v) = λv. Then v is called an eigenvector of T , and λ is called the eigenvalue of T corresponding to v.
51
Give the definition of Eigenvectors/Eigenvalues of a Matrix
Let A be an n×n matrix over K. Suppose that, for some non-zero column vector v ∈ Kn,1 and some scalar λ ∈ K, we have Av = λv. Then v is called an eigenvector of A, and λ is called the eigenvalue of A corresponding to v.
52
What is The Characteristic Equation/Polynomial for a Matrix
For an n × n matrix A, the equation det(A − xIn ) = 0 is called the characteristic equation of A, and det(A − xIn) is called the characteristic polynomial of A.