C1W2 Errata

Not official, and note things get revised frequently. But here are the mistakes I noted in an otherwise great course. As of Apr 5 2024.

Video: The rank of a matrix in general

- at 0:39, the transcript is wrong. It should say “In system 1, you can check out that all the equations are linearly INdependent.”"

Row echelon form in general

The row echelon form matrix is incorrect. Given a matrix 1 1 2 | 3 -3 -1 | 2 -1 6

the row echelon form should be: 1 1 2 |0 -6 -7 |0 0 5.5

Related: C1_W2 - Row Echelon form in general - Wrong Row Echelon form

Video: The Gaussian Elimination Algorithm

Given the augmented matrix 2 -1 1 1 | 2 2 4 -2 | 4 1 0 -1.

The mistake is at 3:00, in the bottom right, when subtracting 4*row1 from row 3. The last element is calculated wrong. The new row3 should be 0 3 -2 -3. The rest of the math only works if you accept this mistake. The correct result is 1 0 0 0 | 0 1 0 -1 | 0 0 1 0

resulting in a = 0, b = -1, c = 0.

Related: Re: The Gaussian Elimination Algorithm - How did they get the value 2 in the pivot example?

Programming Assignment: Gaussian Elimination

The only current issue I see is in the “5 - Back substitution” example. The first operation on R0, where 1/2 R1 is subtracted from R0 is incorrect.

| 1 -1 1/2 1/2 | - (1/2 | 0 1 1 -1| )

| 1 -1 1/2 1/2 | - |0 1/2 1/2 -1/2|

| 1 -3/2 0 1 |

hth,

Jeremy