Engineering Analysis ENG 3420 Fall 2009Lecture 15LU FactorizationGauss Elimination as LU FactorizationCholesky FactorizationEngineering Analysis ENG 3420 Fall 2009Dan C. MarinescuOffice: HEC 439 BOffice hours: Tu-Th 11:00-12:0022Lecture 15Lecture 15Last time: Discussion of pivotingTri-diagonal system solverExamplesToday:Symmetric, Hermitian, Positive matrices Matrix multiplicationNon-commutativeAssociativeThe transpose of a product of two matricesLU Factorization (Chapter 10)Cholesky decompositionNext TimeMidterm - Exams are open book and open notes. Computers are not allowed. There is no make-up for any exam; if you miss an exam and have a good reason you need to go to the Academic Services and bring a letter stating that your absence was motivated and then discuss with the instructor.LU FactorizationLU factorization involves two steps:Decompose the [A] matrix into a product of: a lower triangular matrix [L] with 1 for each entry on the diagonal.and an upper triangular matrix [USubstitution to solve for {x}Gauss elimination can be implemented using LU factorization The forward-elimination step of Gauss elimination comprises the bulk of the computational effort.LU factorization methods separate the time-consuming elimination of the matrix [A] from the manipulations of the right-hand-side [b].Gauss Elimination as LU FactorizationTo solve [A]{x}={b}, first decompose [A] to get [L][U]{x}={b}MATLAB’s lu function can be used to generate the [L] and [U] matrices:[L, U] = lu(A)Step 1 solve [L]{y}={b}; {y} can be found using forward substitution.Step 2 solve [U]{x}={y}, {x} can be found using backward substitution.In MATLAB:[L, U] = lu(A)d = L\bx = U\dLU factorization requires the same number of floating point operations (flops) as for Gauss elimination.Advantage once [A] is decomposed, the same [L] and [U] can be used for multiple {b} vectors.Cholesky FactorizationA symmetric matrix a square matrix, A, that is equal to its transpose: A = AT (T stands for transpose).The Cholesky factorization based on the fact that a symmetric matrix can be decomposed as: [A]= [U]T[U]The rest of the process is similar to LU decomposition and Gauss elimination, except only one matrix, [U], needs to be stored.Cholesky factorization with the built-in chol command:U = chol(A)MATLAB’s left division operator \ examines the system to see which method will most efficiently solve the problem. This includes trying banded solvers, back and forward substitutions, Cholesky factorization for symmetric systems. If these do not work and the system is square, Gauss elimination with partial pivoting is
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