If P1 and P2 are permutation matrices, so is P1P2. This still has the rows of I in some
Chapter 1, Problem 1.5.42(choose chapter or problem)
If P1 and P2 are permutation matrices, so is P1P2. This still has the rows of I in some order. Give examples with P1P2 6= P2P1 and P3P4 = P4P3.
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