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The Gauss-Seidel method is a remarkably easy to implement Iterative method for solving systems of linear equations based on the Jacobi iteration method. It is easier to implement (can be done in only 10s of lines of C code) and it is generally faster than the Jacobi iteration, but its convergence speed still makes this method only of theoretical interest.

# Brief reminder of Jacobi's iteration method

If you are already familiar with Jacobi's iteration method you can skip this paragraph as long as you give a glance at the notations I've chosen.

Suppose you wish to solve a (n,n) linear system. This system can be written :

A * x = b
where A is a (n,n) matrix, x and b are (n,1) column vectors.

The Jacobi iteration method is to write the matrix A in the form

A = D + N
where D is a diagonal matrix and N a matrix with only 0's on the diagonal. A rewrite of the above formulae with A's decomposition gives
(D + N) * x = b, hence D * x = b - N * x
The idea behind iteration is that you start with a random (it should be chosen as close as possible to the solution to speed up iteration time) estimation x0 of the solution and apply a recursive formulae xn+1 = f(xn). The function f() should be chosen so that the sequence (x0, x1, ...) has the solution for limit.

The above formulae suggests you can compute the "new" value of x given its "old" value. Therefore it gives the following iteration formulae :

xn+1 = D-1 * b - D-1 * N * xn
The interesting thing about this formulae is that D-1 * b and D-1 * N are constants throughout the iteration.

# Gauss-Seidel

In order to compute xn+1 from xn, one would probably allocate new space where to store the elements of the vector and compute them one by one. Then to complete the iteration, xn+1 must be copied where xn was and the situation is ready to start over.
xn+1 `<-` D-1 * b - D-1 * N * xn
xn `<-` xn+1

Gauss-Seidel's approach is to compute each element of xn+1 and directly update xn with that value. This eliminates the need for a separate memory location. The calculation is done "in place" instead of being copied to another location.
Let xk be the k_th element of vector x :

xnk `<-` (D-1 * b)k - (D-1 * N * xn)k

# Strengths and drawbacks

Gauss-Seidel iteration has the same drawbacks than Jacobi iteration, including but not limited to :
• The matrix D is not always invertible. One must check that the values on the diagonal are non-zero before starting the iteration because it can lead to unpredictable results.
• The spectral radius of the matrix D-1 * N must have a modulus < 1 for the iteration to work correctly. Think of the spectral radius (the largest value of the set of eigenvalue modules) as being the greatest scalar factor that can be applied to a vector. If it is greater than 1, iteration on the corresponding eigenvector will result in an infinite limit.
But tests I've conducted on a fair number of random diagonally dominant matrices representing linear systems show that this method is between twice or thrice faster than the traditional Jacobi iteration method.

Besides such matrices are frequently encountered when applying finite difference methods, for example when trying to solve boundary value differential problems.

# Conclusion

Jacobi iteration and Gauss-Seidel iteration are good numerical analysis reference methods to evaluate the performance of other linear system solving schemes. Besides Gauss-Seidel iteration turns out to be very important in multigrid relaxation methods.

The Gauss-Seidel method has a faster convergence speed than Jacobi's iteration method (about twice faster) and is even easier to implement. It takes only tens of line of C code to program it. This makes it a choice method for computer scientists willing to test the validity of a model. However I really urge them to use a better relaxation method if they intend to do something at least a little serious because typical convergence speed is so slow that it makes this method only of theoretical interest. It takes o(p*J2) iterations to reduce the error by a factor 10p of a (J,J) linear system with diagonally dominant matrices.

##### Numerical Recipes in C - 19.5 Relaxation methods for boundary value problems

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