Math 5630: Iterative Methods for Systems of Equations Hung Phan, UMass Lowell March 22, 2018

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1 1 Linear Systems Math 5630: Iterative Methods for Systems of Equations Hung Phan, UMass Lowell March, 018 Consider the system 4x y + z = 7 4x 8y + z = 1 x + y + 5z = 15. We then obtain x = 1 4 (7 + y z) y = 1 8 (1 + 4x + z) z = 1 5 (15 + x y). Jacobi Iteration: Given (x k, y k, z k ), the next iteration is defined by x k+1 = 1 4 (7 + y k z k ) y k+1 = 1 8 (1 + 4x k + z k ) which can be easily written in matrix form z k+1 = 1 5 (15 + x k y k ), x k+1 x k y k+1 = M y k + c. z k+1 z k Gauss-Seidel Iteration: Given (x k, y k, z k ), the next iteration is defined by x k+1 = 1 4 (7 + y k z k ) y k+1 = 1 8 (1 + 4x k+1 + z k ) z k+1 = 1 5 (15 + x k+1 y k+1 ). To derive matrix form for Gauss-Seidel iteration, suppose the system is a 11 x + a 1 y + a 13 z = b 1 a 1 x + a y + a 33 z = b, a 31 x + a 3 y + a 33 z = b 3 Then Gauss-Seidel is a 11 x k+1 = a 1 y k a 13 z k +b 1 a y k+1 = a 1 x k+1 a 3 z k +b. a 33 z k+1 = a 13 x k+1 a 3 y k+1 +b 3 We rewrite This means, a 11 x k+1 = a 1 y k a 13 z k +b 1 a 1 x k+1 + a y k+1 = a 13 z k +b. a 31 x k+1 + a 3 y k+1 +a 13 z k+1 = b 3 a x k+1 0 a 1 a 13 x k b 1 a 1 a 0 y k+1 = 0 0 a 3 y k + b. a 31 a 3 a 33 z k z k b 3 In general, consider the linear system Ax = b. We decompose A = L + D + U, where L, D, U are the lower triangular, diagonal, and upper triangular portion of A. Then Jacobi method can be derived as Dx = (L + U)x + b x k+1 = D 1 (L + U)x k + D 1 b, and Gauss-Seidel method can be derived as (L + D)x = Ux + b x k+1 = (L + D) 1 Ux k + (L + D) 1 b. In Matlab/Julia, we can obtain L and U by L=tril(A,-1) and U=triu(A,1). 1

2 Successive Over Relaxation (SOR) with parameter ω (1, ): at each entry, this method interpolates the Gauss-Seidel entry and the old entry as follows: which implies Next, which implies a 11 x k = a 1 y k a 1 z k + b, and x k+1 = (1 ω)x k + ω x k, a 11 x k+1 = (1 ω)a 11 x k ωa 1 y k ωa 13 z k + ωb 1. a ỹ k = a 1 x k+1 a 3 z k + b, and y k+1 = (1 ω)y k + ωỹ k, ωa 1 x k+1 + a y k+1 = (1 ω)a y k ωa 3 z k + ωb Deriving similar formula for z k+1, we can write a x k+1 (1 ω)a 11 ωa 1 ωa 13 ωa 1 a 0 y k+1 = 0 (1 ω)a ωa 3 ωa 31 ωa 3 a 33 z k (1 ω)a 33 Dividing by ω and using x k as a vector, we obtain the SOR equation ( 1 ω D + L) x k+1 = ( ( 1 ω 1)D U) x k + b. x k y k + ω z k b 1 b. b 3 Strictly Diagonally Dominant Matrices Definition 1 A matrix A N N is said to be strictly diagonally dominant if a kk > a ki for k = 1,,..., N. i k Theorem Suppose that A N N is strictly diagonally dominant. Then A is invertible. Proof. Suppose on the contrary that there exists x = (x 1,..., x N ) R N {0} such that Ax = 0. Let j be an index with largest absolute value x j, that means, x j = max x i > 0. The j-th row of Ax = 0 reads as 0 = a j1 x 1 + a j x + + a jn x N a jj x j = a ji x i. i It follows that 0 < a jj x j = a 1i x i a ji x i a ji max x i < a jj x j. i which is a contradiction. Thus, A is invertible. Theorem 3 Suppose A = [a ij ] R N N is strictly diagonally dominant and b = (b i ) R N. Then the Jacobi iteration will converge to the unique solution of Ax = b for any starting point. Proof. We use the matrix form of Jacobi iterates x k+1 = Mx k + c where M := D 1 (L + U), c := D 1 b.

3 We will show that M < 1. Indeed, each absolute row sum in M is smaller than 1, for example, row 1 of M reads as M 1 = 1 a 11 [ 0, a1,..., a 1N ] M1 1 = 1 a 11 a 1i < 1. i 1 This implies M < 1. Theorem 4 Suppose A = [a ij ] R N N is strictly diagonally dominant and b = (b i ) R N. Then the Gauss-Seidel iterates converges to the unique solution of Ax = b for any starting point. Proof. The Gauss-Seidel in matrix form reads as x k+1 = Mx k + c where M = (D + L) 1 U, c = (D + L) 1 b. We will show that M < 1. Take x R N {0} and let y = Mx, i.e., (D + L)y = Ux. Consider any row j, we have a jj y j + k<j a jk y k = k>j a jk x k. Let j be such that y = y j. Then the left and right hand side satisfy LHS a jj y j a jk y k a jj y j a jk y j = a jj a jk y, k<j k<j k<j RHS a jk x j a jk x. k>j k>j It follows that where γ = max j y k>j a jk a jj k<j a jk x γ x, k>j a jk a jj k<j a jk < 1. This proves M < 1. 3 Positive Definite Matrices Definition 5 A complex matrix A = [a ij ] C N N is said to be Hermitian if A = A where A := [a ij ] is the conjugate transpose. In the formula a ij is the complex conjugate, i.e., if a ij = x + iy C then a ij = x iy. Theorem 6 Let A C N N be Hermitian and positive definite, and ω (0, ). Then the SOR method for solving Ax = b converges to a unique solution for any starting point. Proof. Recall that SOR method is written as ( 1 ω D + L) x k+1 = ( ( 1 ω 1)D U) x k + b. 3

4 Define Q = ( 1 ω D + L) and G = Q 1 ( ( 1 ω 1)D U). We will show that the spectrum radius ρ(g) < 1. Let (λ, x) be a (possibly complex) eigenpair. Define y := (I G)x = (1 λ)x. Notice that I G = Q 1[ Q ( 1 ω 1)D + U ] = Q 1 A. So, Qy = Ax y = Q 1 Ax. Next, (Q A)y = Ax AQ 1 Ax = A(I Q 1 A)x = AGx = λax. It follows that So Qy, y = Ax, y = Ax, (1 λ)x, and y, (Q A)y = y, λax = (1 λ)λ x, Ax. 1 ω Dy, y + Ly, y = (1 λ) Ax, x ( 1 ω 1) Dy, y y, Uy = (1 λ)λ x, Ax. Since Ly, y = y, Uy, we add two equations ( ω 1) Dy, y = (1 λ ) Ax, x. Since x 0, y = Q 1 Ax 0. So Dy, y > 0, Ax, x > 0 and that ω 1 > 0. Thus, we have 1 λ > 0, which means λ < 1. The proof is complete. 4 Picard Iterates for Nonlinear Systems { { x = f xk+1 = f(x k, y k ) Suppose we solve the system by the Picard iterates y = g y k+1 = g(x k, y k ). Suppose f and g are continuously differentiable around a fixed point. Then by mean value theorem [ xk+1 x f(xk, y = k ) f f = x (u 1, v 1 ) f ] y(u 1, v 1 ) xk x y k+1 y g(x k, y k ) g g x(u, v ) g y(u,, v ) y k y where (u 1, v 1 ) and (u, v ) are some points on the segment connecting and (x k, y k ). From this equation, we easily derive the following result. { x = f Theorem 7 Let be a fixed point of where f and g are continuously differentiable. y = g Suppose for all points (u 1, v 1 ), (u, v ) around, the matrix [ f x (u 1, v 1 ) f y(u ] 1, v 1 ) g x(u, v ) g y(u has infinity norm less than 1., v ) Then for starting point (x 0, y 0 ) close to, the Picard iterate converges to. Definition 8 (Jacobian matrix) The matrix [ f x f y(x, ] y) g x(x, y) g y(x,, or equivalently, y) is called the Jacobian matrix of the pair of functions (f, g). f x x f y y, 4

5 Example 9 Use Picard iteration to solve the system of nonlinear equations x x y = 0 x + 4y 4 = 0. Solution. The problem is to find the intersection of a parabola and an ellipse. y = x x 0.5 x 4 + y 1 = 1 There are solutions: one near ( 0., 1) and one near (1.9, 0.3). First, convert the system to x = x y x k+1 = x k y k y = x 4y and use the iterate + 8y + 4 y k+1 = x k 4y k + 8y k Then with the starting point (0, 1), we find the solution ( 0.146, ). cannot detect the second solution. To see why it is the case, we define f = x y and consider the Jacobian matrix of (f, g) J = and g = x 4y + 8y + 4, 8 x 1 x 4 y + 1 But this iterate Consider the neighborhood Ω = { 0.4 x 0, 0.8 y 1. } around the first solution (see picture), we have f x + f y = x < 1, g x(x, y) + g y(x, y) = x 4 + y < 1. That implies for all Ω, the infinity norm of the Jacobian J < 1. By Theorem 7, the Picard iterate converges to a solution for all starting point in Ω. On the other hand, if we consider the neighborhood Ω = { 1.8 x, 0. y 0.4 } around the second solution, then we check that f x + f y = x > > 1. This means, the Picard iterate might not converge for starting point in Ω. In order to find the second solution, we will use the Newton method below. 5

6 5 Newton Method for Nonlinear Systems { 0 = f, Consider the system. To develop the Newton method for solving this system, we 0 = g. consider small changes in the function near the point (x 0, y 0 ). We have f f(x 0, y 0 ) f x (x 0, y 0 )(x x 0 ) + f y (x 0, y 0 )(y x 0 ) g g(x 0, y 0 ) x (x 0, y 0 )(x x 0 ) + y (x 0, y 0 )(y x 0 ) If is a solution, then f x x0 J(x 0, y 0 ) := x (x 0, y 0 ) y x 0 x (x 0, y 0 ) If the Jacobian matrix is nonsingular, then we can solve for as follows [ xk xk f(xk, y k ) xk Given, solve J(x y k, y k ) = for k y k g(x k, y k ) y k xk+1 xk xk Set = +, then repeat. y k+1 y k y k { 0 = f, Theorem 10 Consider the system f y (x 0, y 0 ) x x0 f(x0, y 0 ) y (x y x 0, y 0 ) 0 g(x 0, y 0 ) x. So the Newton method can be described y. If the Jacobian of (f, g) is invertible in a neighborhood of a solution, then Newton s method converges when the starting point is close to the 0 = g. solution. Example 11 Use Newton method to solve the system of nonlinear equations f = x x y = 0 g = x + 4y 4 = 0. x 1 Solution. The Jacobian is J =. To check if J is invertible, we consider the determinant x 8y det J = 8y(x ) + x. For the solution near ( 0., 1), we have For the solution near (1.9, 0.3), we have det J det J( 0., 1) = 8(( 0.) ) + ( 0.) 0. det J det J(1.9, 0.3) = 8((1.9) ) + (0.3) 0. So, we can apply Newton method near both solutions. The Newton iterates read as xk+1 xk = J(x k, y k ) 1 f(xk, y k ). g(x k, y k ) y k+1 y k Use Newton method with (x 0, y 0 ) = (0, 1), we obtain the solution ( 0.146, ). Use Newton method with (x 0, y 0 ) = (, 0), we obtain the solution ( , ). ]. 6