Why does swapping rows in the original matrix require swapping columns in the inverse matrix?
Conditions
- The original matrix and its inverse satisfy the identity .
- The rows of the original matrix are permuted to form a new matrix.
- The goal is to find the inverse of the new matrix.
Reasoning, step by step
- Recall that matrix multiplication computes entries via row-column inner products.
- Identify the new position of a specific row in the permuted matrix.
- Determine which column in the inverse matrix originally paired with that row to produce a 1 on the diagonal.
- Move that column to the new position corresponding to the row's new index.
- Verify that all other row-column pairings yield 0 off the diagonal and 1 on the diagonal.
Example
In the video, the first row of , , originally pairs with the first column of , , to produce the entry of the identity matrix. When this row moves to the second position in the new matrix , the column must move to the second position in the new inverse matrix so that their inner product produces the entry of the identity matrix.
Common misconceptions
- Ignoring that columns must move when rows move, and attempting to use the original column order of the inverse matrix for pairing.
- Believing that the product of the new matrix and the original inverse matrix will still be the identity matrix.
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