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Linear Algebra, Theory And Applications, 2012a

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122 ROW OPERATIONS<br />

28. Let {v 1 , ··· , v n−1 } be vectors in F n . Describe a systematic way to obtain a vector v n<br />

which is perpendicular to each of these vectors. Hint: You might consider something<br />

like this<br />

⎛<br />

⎞<br />

e 1 e 2 ··· e n<br />

v 11 v 12 ··· v 1n<br />

det ⎜<br />

⎟<br />

⎝ . .<br />

. ⎠<br />

v (n−1)1 v (n−1)2 ··· v (n−1)n<br />

where v ij is the j th entry of the vector v i .Thisisalotlikethecrossproduct.<br />

29. Let A be an m × n matrix. Then ker (A) is a subspace of F n . Is it true that every<br />

subspace of F n is the kernel or null space of some matrix? Prove or disprove.<br />

30. Let A be an n×n matrix and let P ij be the permutation matrix which switches the i th<br />

and j th rows of the identity. Show that P ij AP ij produces a matrix which is similar<br />

to A which switches the i th and j th entries on the main diagonal.<br />

31. Recall the procedure for finding the inverse of a matrix on Page 48. It was shown that<br />

the procedure, when it works, finds the inverse of the matrix. Show that whenever<br />

the matrix has an inverse, the procedure works.

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