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Linear Algebra, Theory And Applications, 2012a

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112 ROW OPERATIONS<br />

At this point it is clear the rank is 2. This is because every column is in the span of the<br />

first two and these first two columns are linearly independent.<br />

Example 4.2.6 Find the rank of the following matrix and identify columns whose linear<br />

combinations yield all the other columns.<br />

⎛<br />

⎝ 1 2 1 3 2<br />

⎞<br />

1 2 6 0 2 ⎠ (4.2)<br />

3 6 8 6 6<br />

Take (−1) times the first row and add to the second and then take (−3) times the first<br />

row and add to the last row. This yields<br />

⎛<br />

⎝ 1 2 1 3 2<br />

⎞<br />

0 0 5 −3 0 ⎠<br />

0 0 5 −3 0<br />

Now multiply the second row by 1/5 andadd5timesittothelastrow.<br />

⎛<br />

⎝ 1 0 2 0 1 1 3 −3/5 2<br />

0<br />

⎞<br />

⎠<br />

0 0 0 0 0<br />

Add (−1) times the second row to the first.<br />

⎛<br />

⎝ 1 2 0 ⎞<br />

18<br />

5<br />

2<br />

0 0 1 −3/5 0 ⎠ (4.3)<br />

0 0 0 0 0<br />

It is now clear the rank of this matrix is 2 because the first and third columns form a<br />

basis for the column space.<br />

The matrix (4.3) is the row reduced echelon form for the matrix (4.2).<br />

4.3 The Row Reduced Echelon Form<br />

The following definition is for the row reduced echelon form of a matrix.<br />

Definition 4.3.1 Let e i denote the column vector which has all zero entries except for the<br />

i th slot which is one. An m×n matrix is said to be in row reduced echelon form if, in viewing<br />

successive columns from left to right, the first nonzero column encountered is e 1 and if you<br />

have encountered e 1 , e 2 , ··· , e k , thenextcolumniseithere k+1 or is a linear combination<br />

of the vectors, e 1 , e 2 , ··· , e k .<br />

For example, here are some matrices which are in row reduced echelon form.<br />

⎛<br />

⎝ 0 1 3 0 3<br />

⎞ ⎛<br />

0 0 0 1 5 ⎠ , ⎝ 1 0 3 −11 0<br />

⎞<br />

0 1 4 4 0 ⎠ .<br />

0 0 0 0 0 0 0 0 0 1<br />

Theorem 4.3.2 Let A be an m × n matrix.<br />

determined by a simple process.<br />

Then A has a row reduced echelon form

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