# Let $T$ be a $4 \times 4$ matrix with real entries

Let $T$ be $4\times 4$ matrix with real entries. Suppose $T^5=0$. Then which of the following is necessarily true?

(A) $T$ is the zero matrix.
(B) $T$ need not be the zero matrix, but $T^2$ is the zero matrix.
(C) $T^2$ need not be the zero matrix, but $T^3$ is the zero matrix.
(D) $T^3$ need not be the zero matrix, but $T^4$ is the zero matrix.

How can I tackle the above problem? Any help will be appreciated.Thanks in advance for your time.

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Do you know of the minimal polynomial of a matrix, or the Cayley-Hamilton theorem? –  Andres Caicedo Jan 20 '13 at 6:41
The answer is here: en.wikipedia.org/wiki/Nilpotent_matrix. Look for the line: "The degree of an $n\times n$ nilpotent matrix is always less than or equal to $n$," and the phrase "canonical nilpotent matrix." The proof is another story. –  Anon Jan 20 '13 at 6:44
Related: math.stackexchange.com/q/108422 –  Jonas Meyer Jan 20 '13 at 6:54
Thanks for the link.It's been useful. Now degree of $4 \times 4$ nilpotent matrix $T$ must be $\leq 4$. From here i can not decide about which of the given options can be correct.Can you help me a little bit more? –  user53386 Jan 20 '13 at 6:55
@Anon I have got your point sir. Surely $(B)$ will be the correct choice. So here in the aforementioned problem ,i guess $(D)$ is the right choice.Am i right sir? –  user53386 Jan 20 '13 at 7:08

A nilpotent matrix's degree should be less that or equal to its dimension, so your matrix's degree is less or equal to 4 so :

statement (D) is correct and all other statements are false, because of this example:

$N$ = \begin{bmatrix} 0 & 2 & 1 & 6\\ 0 & 0 & 1 & 2\\ 0 & 0 & 0 & 3\\ 0 & 0 & 0 & 0 \end{bmatrix}

$N^2$ = \begin{bmatrix} 0 & 0 & 2 & 7\\ 0 & 0 & 0 & 3\\ 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0 \end{bmatrix} $N^3$ = \begin{bmatrix} 0 & 0 & 0 & 6\\ 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0 \end{bmatrix} $N^4$ = \begin{bmatrix} 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0 \end{bmatrix} $N^5$ = \begin{bmatrix} 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0 \end{bmatrix}

So $T$ need not to be zero then statement (A) is false

$T^2$ need not be zero then statement (B) is false

$T^3$ need not be zero then statement (c) is false

every $4\times 4$ matrix that is nilpotent should have a degree less than or equal to $4$, $T$ is nilpotent, then $T$'s degree is less than or equal to $4$, then $T^4$ need to be zero, then statement(D) is true

ATTENTION:

for disproving a statement, a negative example suffices, so for disproving statements (A), (B) and (C) the example works But for proving statement (D) you should first prove this theorem:

 A is a n×n matrix, Am=0 for some positive integer m. Show that An=0.


then you could see that (D) is true.

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Thanks a lot sir for the detailed discussion of the problem .I have got it now. –  user53386 Jan 22 '13 at 4:28