Are Column Vectors in AB Unchanged if Linearly Independent?

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SUMMARY

The discussion confirms that if a 3x3 matrix AB has a rank of 2, any two linearly independent vectors in the column space of AB are also present in the column space of matrix A. This is established by the relationship where any vector x in the column space of AB can be expressed as x = ABy, indicating that x must also belong to the column space of A. However, the reverse is not true; vectors in the column space of A may not necessarily be found in the column space of AB.

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If you have a 3x3 matrix AB of rank 2, and you are asked to find two linearly independent vectors in the column space of A, are those the same vectors in the column space of AB? I figure it is since if the vectors are linearly independent they wouldn't change. Am I right?
 
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That's right. If x is in the column space of AB, then x = ABy for some vector y. But since x = A(By), x is in the column space of A. Thus, if you can find two linearly independent vectors in col AB, you've found two linearly independent vectors in col A.

Be careful, as the opposite isn't true: it's not necessarily true that if x is in the column space of A, then it's in the column space of AB.
 

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