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Iterative Methods for Solving Ax = b - The SOR Method

David M. Strong

A third iterative method, called the Successive Overrelaxation (SOR) Method, is a generalization of and improvement on the Gauss-Seidel Method. Here is the idea:

For any iterative method, in finding x(k+1) from x(k), we move a certain amount in a particular direction from x(k) to x(k+1). This direction is the vector x(k+1)x(k), since x(k+1) = x(k) + (x(k+1)x(k)). If we assume that the direction from x(k) to x(k+1) is taking us closer, but not all the way, to the true solution x , then it would make sense to move in the same direction x(k+1)x(k), but farther along that direction.

Here is how we derive the SOR Method from the Gauss-Seidel Method. First, notice that we can write the Gauss-Seidel equation as


so that

We can subtract x(k) from both sides to get

Now think of this as the Gauss-Seidel correction (x(k+1)x(k))GS. As suggested above, it turns out that convergence x(k) x of the sequence of approximate solutions to the true solution is often faster if we go beyond the standard Gauss-Seidel correction. The idea of the SOR Method is to iterate

where, as we just found,

and where generally 1 < ω < 2. Notice that if ω = 1 then this is the Gauss-Seidel Method.

Written out in detail, the SOR Method is

We can multiply both sides by matrix D and divide both sides by ω to rewrite this as

then collect the x(k+1) terms on the left hand side to get


When we solve for x(k+1), we get

Notice that the SOR Method is also of the form x = Bx + , so the general convergence analysis on page 6 also applies to the SOR Method, as does the more specific analysis on page 7 for the Jacobi and Gauss-Seidel Methods. The iteration matrix B that determines convergence of the SOR Method is


so optimal convergence is achieved by choosing a value of ω that minimizes

As we did earlier for the Jacobi and Gauss-Seidel Methods, we can find the eigenvalues and eigenvectors for the 2 x 2 SOR Method B matrix. However, because this is quite a bit more complicated, we do not derive these expressions here. Rather, we leave it as Exercise 18 (next page) for the ambitious student or the challenging instructor.

In practice, we typically use a computer to perform the iterations, so we need implementable algorithms in order to use these methods for n x n systems. We conclude by giving one possible set of algorithms for finding element xi(k+1) given x1(k), x2(k), … , xn(k). Although these particular algorithms are not quite optimally efficient, writing the algorithms this way makes more obvious the slight but important differences among the three methods.

Method Algorithm for performing iteration k + 1:
for i = 1 to n do:

David M. Strong, "Iterative Methods for Solving [i]Ax[/i] = [i]b[/i] - The SOR Method," Convergence (July 2005)