# If A is a nilpotent matrix of index 2, then find the value of $A{(I + A)^n}$ for any positive integer n.

A. ${A^{ - 1}}$

B. $A$

C. ${A^n}$

D. ${I_n}$

Last updated date: 20th Mar 2023

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Answer

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Hint: Nilpotent matrix A, means some power of A is equal to the zero matrix.

Complete step-by-step answer:

Given A is a nilpotent matrix of index 2.

${A^2} = 0$

${A^3} = 0$

${A^3} = 0....$

${A^n} = 0$

Now, we have to find the value of $A{(I + A)^n}$

$ \Rightarrow A{(I + A)^n} = A{[^n}{C_0}{I^n}{ + ^n}{C_1}{I^{n - 1}}A{ + ^n}{C_2}{I^{n - 2}}{A^2} + ....{ + ^n}{C_n}{I^0}{A^n}]$

$ \Rightarrow A{(I + A)^n} = A\left[ {I + nA} \right]$

$ \Rightarrow A{(I + A)^n} = AI + n{A^2}$

$ \Rightarrow A{(I + A)^n} = A$

$\therefore $ The value of $A{(I + A)^n} = A$

Note: A nilpotent matrix is a square matrix N, such that ${N^k} = 0,$ for some positive integer k. The smallest such k is sometimes called the index of N.

Complete step-by-step answer:

Given A is a nilpotent matrix of index 2.

${A^2} = 0$

${A^3} = 0$

${A^3} = 0....$

${A^n} = 0$

Now, we have to find the value of $A{(I + A)^n}$

$ \Rightarrow A{(I + A)^n} = A{[^n}{C_0}{I^n}{ + ^n}{C_1}{I^{n - 1}}A{ + ^n}{C_2}{I^{n - 2}}{A^2} + ....{ + ^n}{C_n}{I^0}{A^n}]$

$ \Rightarrow A{(I + A)^n} = A\left[ {I + nA} \right]$

$ \Rightarrow A{(I + A)^n} = AI + n{A^2}$

$ \Rightarrow A{(I + A)^n} = A$

$\therefore $ The value of $A{(I + A)^n} = A$

Note: A nilpotent matrix is a square matrix N, such that ${N^k} = 0,$ for some positive integer k. The smallest such k is sometimes called the index of N.

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