# Simple Matrix Subtraction Confusion

I am looking at this answer that is causing me confusion but I am not able to comment on it to gain clarification. Within it A is a matrix declared as follows:

$$A= \left[\begin{array}{ccc} 6 & -4 & 0 \\ 4 & -2 & 0 \\ -1 & 0 & 3\end{array}\right]$$

The person who answers the question proceeds to claim that $$A - 2I = \begin{bmatrix} 4 & -4 & 0 \\ 4 & -4 & 0 \\ -1 & 0 & 3\end{bmatrix}$$

Is it not supposed to be

$$A - 2I = \begin{bmatrix} 4 & -4 & 0 \\ 4 & -4 & 0 \\ -1 & 0 & 1\end{bmatrix}$$

and if so why not?

• @MatthewLeingang Apr 24, 2020 at 0:58
• It appears that the person made a mistake. The lower right component should be $3-2=1$ Apr 24, 2020 at 1:01
• With that being said, I then require further clarification in regards to the answer. How would I get there attention? Can I tag them in this post? @J.W.Tanner Apr 24, 2020 at 1:03
• I'm not sure how you would get attention, but I left a comment on that answer Apr 24, 2020 at 1:08
• You and @J.W.Tanner are absolutely correct. It was a simple typo. I apologize for the confusion. Apr 26, 2020 at 15:43

You are correct. Because $$I_3=\begin{pmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{pmatrix},$$ each diagonal component should have $$2$$ subtracted from it; the $$(3,3)$$ entry has been missed.