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What is the permutation matrix representation of the cyclic row rearrangement in the video?

The cyclic row rearrangement where rows 3, 1, and 2 of the original matrix form the new matrix can be represented by left-multiplying the original matrix by a specific 3×33 \times 3 permutation matrix. This permutation matrix has a 1 in the (1,3)(1,3), (2,1)(2,1), and (3,2)(3,2) positions, and 0 elsewhere. It acts as an operator that shifts the rows of the matrix it multiplies according to the specified cycle.

Conditions

  • The new matrix is formed by taking the 3rd, 1st, and 2nd rows of the original matrix in that order.
  • The permutation is applied via left-multiplication.

Reasoning, step by step

  1. Identify the row mapping: row 1 goes to row 2, row 2 goes to row 3, and row 3 goes to row 1.
  2. Construct the permutation matrix PP such that Pij=1P_{ij} = 1 if row jj of the original becomes row ii of the new matrix.
  3. Place 1s at positions (1,3)(1,3), (2,1)(2,1), and (3,2)(3,2) in the 3×33 \times 3 matrix.
  4. Fill the remaining entries with 0.
  5. Verify that PAPA produces the desired row rearrangement.

Example

For the matrix A=[abcdefghi]A = \begin{bmatrix} a & b & c \\ d & e & f \\ g & h & i \end{bmatrix}, the new matrix is N=[ghiabcdef]N = \begin{bmatrix} g & h & i \\ a & b & c \\ d & e & f \end{bmatrix}. The permutation matrix is P=[001100010]P = \begin{bmatrix} 0 & 0 & 1 \\ 1 & 0 & 0 \\ 0 & 1 & 0 \end{bmatrix}. Multiplying PAPA yields [001100010][abcdefghi]=[ghiabcdef]=N\begin{bmatrix} 0 & 0 & 1 \\ 1 & 0 & 0 \\ 0 & 1 & 0 \end{bmatrix} \begin{bmatrix} a & b & c \\ d & e & f \\ g & h & i \end{bmatrix} = \begin{bmatrix} g & h & i \\ a & b & c \\ d & e & f \end{bmatrix} = N.

Common misconceptions

  • Confusing left-multiplication (which permutes rows) with right-multiplication (which permutes columns).
  • Assuming a three-cycle permutation can be represented by a single elementary row-swap matrix.

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Answers are generated from source material and independently checked. Consult the original video or creator if something is unclear.