## Thursday, February 18, 2010

### Solving Linear Systems

In school you probably learnt how to solve systems of linear equations with techniques like Gaussian elimination, and Row-Reduced Echelon Form (RREF). However a simple, brute-force way to solve linear systems on a computer is through iteration.

Say we wish to solve the following equations:
```4x -  y +  z = 7
4x - 8y +  z =-21
-2x +  y + 5z = 15
```
Then we can re-write them as:  We can solve these with Gauss-Seidel iteration just by plugging in the current x,y,z values we calculate from these equations (and begining with an initial estimate.)

Thus, the Gauss-Siedel method in C-code looks something like:
```#include <stdio.h>

int main() {
//a sparse way of representing the equations
float eq;
eq = 7/4.0; eq = 0; eq = 1/4.0; eq= -1/4.0;
eq = 21/8.0; eq = 4/8.0; eq = 0; eq= 1/8.0;
eq = 15/5.0; eq = 2/5.0; eq = -1/5.0; eq= 0;

float x,y,z;
x=1;y=1;z=2; //initial guess

//10 iterations of gauss-seidel
for (int i=0;i < 10;i++) {
x = eq + eq*y + eq*z;
y = eq + eq*x + eq*z;
z = eq + eq*x + eq*y;
printf("%f %f %f\n",x,y,z);
}

return 0;
}```
Producing this output:
```1.500000 3.625000 2.875000
1.937500 3.953125 2.984375
1.992188 3.994141 2.998047
1.999023 3.999268 2.999756
1.999878 3.999908 2.999969
1.999985 3.999989 2.999996
1.999998 3.999999 3.000000
2.000000 4.000000 3.000000
2.000000 4.000000 3.000000
2.000000 4.000000 3.000000
```

Converging towards the solution nicely. (Things don't always converge, only when Ax=B, A is diagonally dominant - but that is another story)

Jacobi iteration does not converge as quickly, but is easy to execute in parallel. With Jacobi iteration you simply use the last iterations x,y,z value instead of updating it.

See? Solving systems of equations is easy.