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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?

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  • $\begingroup$ @MatthewLeingang $\endgroup$
    – jewelltaylor9430
    Apr 24, 2020 at 0:58
  • $\begingroup$ It appears that the person made a mistake. The lower right component should be $3-2=1$ $\endgroup$
    – J. W. Tanner
    Apr 24, 2020 at 1:01
  • $\begingroup$ 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 $\endgroup$
    – jewelltaylor9430
    Apr 24, 2020 at 1:03
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    $\begingroup$ I'm not sure how you would get attention, but I left a comment on that answer $\endgroup$
    – J. W. Tanner
    Apr 24, 2020 at 1:08
  • $\begingroup$ You and @J.W.Tanner are absolutely correct. It was a simple typo. I apologize for the confusion. $\endgroup$
    – Matthew Leingang
    Apr 26, 2020 at 15:43

1 Answer 1

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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.

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