Is an Eigenvector of A also an Eigenvector of A^2?

In summary, to prove that if s is an eigenvector of matrix A, then it is also an eigenvector of matrix A^2, we can use the property As=Ics and apply the matrix twice to the vector s. This results in (AA)s=Ic(As)=(Ic)^2s, which shows that A^2s=Iks for some number k. Another approach is to start from A^2s=AIcs and use the properties of matrices to get A^2s=Iks.
  • #1
evilpostingmong
339
0

Homework Statement


Prove that if s is an eigenvector of matrix A, then it is also an eigenvector
of matrix A^2.

Homework Equations



As=Ics

The Attempt at a Solution


We know that As=Ics.
So AAs=AIcs
A(As)=A(Ics)
A(As)-A(Ics)=0
A(As-Ics)=0
Since As-Ics=0
A(As-Ics)=A(0)=0
 
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  • #2
Looks about right, but perhaps a bit cumbersome. Just apply the matrix twice to the vector s. In your notation:
(AA)s = A(As) = A(Ics) = Ic(As) = (Ic)^2s
Which is all you need.
 
  • #3
I think it is correct, although admittely I didn't check all of it.
There is an easier way.
From the "relevant equations", you see that you have to show that
A^2 s = I k s
for some number k.

If you start from
A^2 s = A I c s,
can you use two properties of matrices to get the A in front of the s and use A s = I c s again?

[edit]Actually xepma has already given you the complete answer :tongue: [/edit]
 
  • #4
Thanks for the help!
 

1. What is linear algebra?

Linear algebra is a branch of mathematics that deals with the study of linear equations, vector spaces, and linear transformations. It involves the use of algebraic operations to solve systems of linear equations and analyze geometric properties of vector spaces.

2. What is a proof in linear algebra?

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To approach a linear algebra proof, you should first carefully read and understand the given statement or theorem. Then, you can use definitions, properties, and theorems from linear algebra to construct a logical argument that supports the given statement. It is important to use clear and concise notation and to show your steps clearly.

4. What are some common techniques used in linear algebra proofs?

Some common techniques used in linear algebra proofs include direct proof, proof by contradiction, and proof by induction. Other techniques include using properties of matrix operations, vector spaces, and linear transformations to demonstrate the validity of a statement.

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Sure, here is an example of a linear algebra proof: Prove that the sum of two even numbers is always even. Solution: Let x and y be two even numbers. By definition, this means that x = 2a and y = 2b, where a and b are integers. Then, the sum of x and y is x + y = 2a + 2b = 2(a + b). Since a + b is also an integer, it follows that the sum of two even numbers is always even.

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