# Linear Algebra

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1. System of Linear Equation Solutions
Every system of linear equations has either NO solutions, 1 solution, or an INFINITE number of solutions
2. Consistent
A system of equations (linear or not - over any space) is CONSISTENT if it is a NON-EMPTY set
3. Inconsistent
A system of equations (linear or not - over any space) is INCONSISTENT if it is an EMPTY set
4. Methods for solving systems of equations
1. Algebraically eliminate variables

2. Simultaneous elimination

3. Gaussian elimination using matrices
5. Row-Echelon Form (1-3)

Reduced Row-Echelon Form (1-4)
1. First NONzero entry of 1st row is a 1 (leading 1)

• 2. Any rows consisting entirely of zeros are grouped together at the bottom on matrix
• 3. First NON zero entry of 2nd row is a 1 (occuring farther to the right than leading 1 in higher row

*4. Each column containing a leading 1 has zeros everyhwere else
6. Homogeneous
A system of linear equations is said to be homogeneous if the constant terms are all zero; the system has the form:

7. Trivial Solution
Every HOMOGENEOUS system of linear equations is CONSISTENT (since all such systems have 0 as a solution. This is called the TRIVIAL solution
8. NON-trivial soluton
If there are any other solutions of a system of linear equations other than the trivial solutions, these are called NON-trivial solutions
9. Theorem:
- Homogeneous Systems
A homogeneous system of linear equations with more unknowns than equations has infinitely many solutions
10. Definition:
- Matrix
- Entries
A matrix is a rectangualr array of numbers

The numbers in the array are called entries
11. Column Matrix aka Column Vector

Row Matrix aka Row Vector
A column matrix is a matrix with only one column

A row matrix is a matrix with only one row
12. Definition:
- Equal Matrices
Two matrices are defined to be equal if they have the same size and their corresponding entries are equal
13. Theorem:
- Properties of a Zero Matrix
A matrix, all of whose entries are zero is called a zero matrix

1. A + 0 = 0 + A = A

2. A - A = 0

3. 0 - A = -A

4. A0 = 0; 0A = 0
14. Square matrix
A matrix with n rows and n columns is called a square matrix
15. Diagonal Matrix

A square matrix in whichn the entries outside the main diagonal are all zero

*tridiagonal matrices are also square matrices
16. Upper Triangular Matrix
A square matrix a, such that aij = 0 if i>j

17. Lower Triangular Matrix
A square matrix such that aij = 0 if i < j

18. Definition:

*matrices of different sizes cannot be added (or subtracted)
19. Definition:
- Matrix Multiplication
Matrix multiplication is a binary operation that takes a pair of matrices, and produces another matrix. The product c of matrices a and b is defined as:

cik = aij * bjk

*the dimensions of the matrices must satisfy (mxn)(nxp) = (mxp)
20. Definition:
- Matrix Transpose
The transpose of the (mxn) matrix a is the (nxm) matrix formed by interchaning the rows and columns such trhat row i becomes column i of the transposed matrix denoted by aT
21. Definition:
- Matrix Trace
The trace of an n-by-n square matrix A - denoted by tr(A) - is defined to be the sum of the elements on the main diagonal (the diagonal from the upper left to the lower right) of A
22. Theorem:
Properties of Matrix Arithmetic
1. A + B = B + A (commutative law of addition)

2. A + (B + C) = (A + B) + C (associative law of addition)

3. A(BC) = AB + AC (associative law of multiplication)

4. A(B + C) = AB + CA / A(B - C) = AB - CA(left distributive law)

5. (B + C)A = BA + CA / (B - C)A = BA - CA(right distributive law)
23. Theorem:
- Identity Matrices
If R is the rref of an nxn matrix A, then either R has a row of zeros or R is the identity matrix I

*Definiton - An identity matrix has the property that if A is a square matrix, then IA = AI = A

Inxn = [aij] where aij = 1 if i = j, aij = 0 if i does not equal j
24. Definition:
- Invertible
- Inverse
- Singular
If A is a square matrix, and if a matrix B of the same size can be found such that AB = BA = I, then A is said to be invertible and B is called an inverse of A denoted by A-1.

If no such matrix B can be found, then A is said to be singular (not invertible).
25. Theorem:
Properties of Inverses
If B and C are both inverses of the matrix A, then B = C.

• Proof:
• Since B is an inverse of A, we have BA = I.
• Multiplying both sides on the right by C gives (BA)C = IC = C.
• But (BA)C = B(AC) = BI = B, so that C =B
26. incomplete -Theorem:
• The matrix
• A = [ a b
• c d ]
• is invertible if ad - bc
27. Theorem:
- Invertible Matrices
• if A and B are invertible martices of the same size, then AB is invertible and
• (AB)-1 = B-1 A-1

• Proof:
• If we can show that (AB)(B-1A-1) = (B-1A-1)(AB) = I,
• then we will have simultaneously shown that the matrix AB is invertible and
• that (AB)-1 = B-1 A-1.
• But (AB)(B-1A-1) = A(BB-1)A-1 = AIA-1 = AA-1 =I.

*A similar argument shows that (B-1A-1)(AB) = I
28. Product of any # of Invertible Matrices
A product of any number of invertible matrices is invertible, and the inverse of the product is the product of the inverses in the reverse order.
 Author: fmativalu ID: 39238 Card Set: Linear Algebra Updated: 2010-10-04 23:48:36 Tags: linear algebra Folders: Description: Math 343 Show Answers: