Debbie's description is absolutly right and I will show you how to
calculate the
reduced row echelon form of a matrix.
To transform a matrix into the RREF it is only allowed to use so called elemental operations (I hope this is the correct English term):
1. Multiply every element of a row or column with a scalar:
|1 4 5| |2 8 10| (*2)
|3 3 1| ~ |3 3 1 |
|5 3 9| |5 3 9 |
2. Add the multiple of a row(column) to another row(column):
I: |1 4 5| |1 4 5|
II: |3 3 1| ~ |0 -9 -14| (II-3*I)
III: |5 3 9| |5 3 9|
3. Swap
rows or
columns
|1 4 5| |3 3 3|
|3 3 1| ~ |1 4 5|
|5 3 9| |5 3 9|
That's
everything that is allowed to do, because these
operations do not change the rank of the matrix.
Now I will describe the algorithm itself, afterwards I will give a short example:
The matrix is called A and its elements are a(i,j) where i = 1..m and j=1..n. The first thing to do is to make a(1,1)=1. If a(1,1)!=0 then it is very easy: Just divide the first row or column by its value. If a(1,1)=0 then you have to swap columns and(or) rows to get a non-zero value there.
The next thing to do is to make the rest of the first row and column to zero. We start with the rows: Every row k=2..m will be multiplied with a(k,1) times the first row.
The same has to be done with the columns (though here it is every column l=2..n must be multiplied with a(1,l) times the first column.
The above algorithm has to be repeated for the (m-1)x(n-1) matrix, leaving the first row and column alone. It has to be repeated till the
RREF is achived.
The number of
non-zero rows or columns is the rank of the
matrix.
Example:
I: |1 2 3| |1 2 3| |1 0 0| |1 0 0|
II: |2 3 4| ~ |0 -1 -2| (II-2*I) ~ |0 -1 -2| ~ |0 1 2|
III: |3 4 5| |0 -2 -4| (III-3*1) |0 -2 -4| ~ |0 -2 -4|
I: |1 0 0| |1 0 0| |1 0 0| |1 0 0|
II: |0 1 2| (*(-1)) ~ |0 1 2| ~ |0 1 0| ~ |0 1 0|
III: |0 -2 -4| |0 0 -4| ~ |0 0 -4| ~ |0 0 1|
The last step is not needed as one can already see from the previous matrix that the rank R(A)=3.
If you want to check if you understood the
algorithm, just use the following example:
|1 2 0 0|
|1 -1 1 0|
|1 0 2 2|
The rank of this matrix is 3. Calculate it yourself!