MTH 215: Introduction to Linear Algebra

Transcription

1 MTH 215: Introduction to Linear Algebra Lecture 5 Jonathan A. Chávez Casillas 1 1 University of Rhode Island Department of Mathematics September 20, 2017

2 1 LU Factorization 2 3 4

3 Triangular Matrices Definition A matrix A = [a ij is called upper triangular if a ij = 0 whenever i > j. Thus the entries below the main diagonal equal 0. where refers to any number A lower triangular matrix is defined similarly, as a matrix for which all entries above the main diagonal are equal to zero.

4 LU Factorization An LU factorization of a matrix A is written A = LU where L is lower triangular matrix and U is upper triangular. We often require L to have only 1 s on the main diagonal. Motivation The LU factorization often helps to quickly solve equations of the form AX = B. An LU factorization can be found for a matrix A provided that the row-echelon form of A can be calculated without interchanging rows.

5 Table of Contents 1 LU Factorization 2 3 4

6 By Inspection Example Determine if the LU factorization of A exists, and if so, find it. [ A = Solution First check that the row-echelon form can be obtained without interchanging rows. [ [ r 2 2r

7 By Inspection Example Determine if the LU factorization of A exists, and if so, find it. [ A = Solution First check that the row-echelon form can be obtained without interchanging rows. [ [ [ r 2 2r 1 r 3 r

8 By Inspection Example Determine if the LU factorization of A exists, and if so, find it. [ A = Solution First check that the row-echelon form can be obtained without interchanging rows. [ [ [ [ r 2 2r 1 r 3 r 1 r 3 +r

9 Solution (continued) You can see from here that the the row-echelon form can be obtained without interchanging rows. We proceed to finding L and U. Assign variables to the unknown entries and multiply. A = [ = [ x 1 0 y z 1 Solving each entry will give us values for the unknown entries. = [ a d e 0 b f 0 0 c [ a d e ax dx + b ex + f ay dy + bz ey + fz + c

10 Solution (continued) [ = [ a d e ax dx + b ex + f ay dy + bz ey + fz + c We see easily that a = 1, d = 1, and e = 2. Continuing to solve the first column gives x = 2, y = 1. The other values are calculated as follows. dx + b = 3 dy + bz = 0 (1)(2) + b = 3 (1)(1) + (1)z = 0 b = 1 z = 1 ex + f = 0 ey + fz + c = 5 (2)(2) + f = 0 (2)(1) + ( 4)( 1) + c = 5 f = 4 c = 1

11 Solution (continued) Therefore L = U = [ x 1 0 y z 1 [ a d e 0 b f 0 0 c You should multiply these and check that they equal A! [ = [ =

12 Table of Contents 1 LU Factorization 2 3 4

13 The following process for finding L and U, called the multiplier method, can be more efficient. Example Find the LU factorization of A = Solution [ First, write A as [ = [ [ Now we will use row operations (except for interchanging rows) to transform the right side into the appropriate L and U.

14 Solution (continued) To do so, we use row operations to remove the entries of A below the main diagonal. For every operation we apply to A (the matrix on the right), we apply the inverse operation to the identity matrix (on the left). This ensures the product remains the same. The first step is to add ( 2) times the first row of A to the second row. To preserve the product, add (2) times the first row to the second row, for the matrix on the left. r 2 + 2r 1 [ [ r 2 2r 1

15 Solution (continued) We proceed in the same way. r 3 + r 1 [ r 3 + r 1 [ r 3 r 2 [ [ [ [ r 3 r 1 r 3 r 1 r 3 r 1 At this point we have a lower triangular matrix L on the left, and an upper triangular matrix U on the right so we are done. You can (and should!) check that this product equals A.

16 Problem Use the multiplier method to verify the LU factorization for A = [ Solution A = [ = [ [ = LU

17 Table of Contents 1 LU Factorization 2 3 4

18 using LU Factorization Suppose we wish to find all solutions X to the system AX = B. The LU factorization of A can assist in this process. Consider the following reduction: AX = B (LU)X = B L(UX) = B LY = B Therefore if we can solve LY = B for Y, then all that remains is to solve UX = Y for X.

19 Example Find all solutions to [ x 1 x 2 x 3 x 4 = [ Solution Using a method of your choice, verify that the LU factorization of A gives L = [ , U = [

20 Solution (continued) Let Y = [ y1 y 2 y 3 The solution is Y = and solve LY = B. [ Now we solve UX = Y [ [ [ y1 y 2 y 3 x 1 x 2 x 3 x 4 = [ = [ 2 2 4

21 Solution (continued) Multiplying and solving (or finding the row-reduced echelon form), the general solution is given by X = t, t R

22 End of lecture for Thursday, September 21, 2017