Prove Symmetric Matrixes Thm: A=0 or Skew Symmetric

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In summary, we need to prove that A is a symmetric matrix and x(transpose)*A*x = 0 for all x (belongs to R^n) if and only if A = 0, and that x(transpose)*A*x = 0 for all x (belongs to R^n) if and only if A is skew symmetric. For the second statement, you can transpose both sides and use the property A(transposed) = -A to show that x(transpose)*A*x = 0 if and only if A is 0. For the first statement, we can prove the sufficiency easily, while for the necessity, we can either write out the explicit quadratic polynomial and plug in all values of x, or assume
  • #1
blue_m
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I need to prove the following.
1. A is a symmetric matrix, and x(transpose)*A*x=0 for all x (belongs to R^n) if and only if A=0.
2. x(transpose)*A*x=0 for all x (belongs to R^n), if and only if A is skew symmetric.
 
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  • #2
I can suggest that for the second one that you transpose both sides
and use the A(transposed) = -A property so that
x(transpose)*A*x = 0 = -(x(tranpose)*A*x)
so those two can equal 0 if and only if A is 0
 
  • #3
For the first one, sufficiency is obvious. For necessity, either, write the explicit quadratic polynomial and plug in all values of x. Only possibility is the zero polynomial. Alternatively, Assume your [itex]A=V\Lambda V^{-1}[/itex] is diagonalizable with some Jordan blocks (possibly with size [itex]k \times k[/itex]). And use the structure of the Jordan blocks.
 

1. What is a symmetric matrix?

A symmetric matrix is a square matrix that is equal to its own transpose. This means that if you flip the matrix along its main diagonal, the elements on either side will be mirror images of each other. In other words, the matrix is unchanged when reflected along its main diagonal.

2. What is a skew symmetric matrix?

A skew symmetric matrix is a square matrix whose transpose is equal to its negative. This means that if you flip the matrix along its main diagonal, the elements on either side will be negative of each other. In other words, the matrix is unchanged when reflected along its main diagonal and multiplied by -1.

3. What is the Prove Symmetric Matrixes Theorem?

The Prove Symmetric Matrixes Theorem states that any square matrix can be decomposed into a sum of a symmetric matrix and a skew symmetric matrix. This means that any matrix can be written as the sum of a matrix that is unchanged when reflected along its main diagonal, and a matrix that is unchanged when reflected along its main diagonal and multiplied by -1.

4. How do you prove the Prove Symmetric Matrixes Theorem?

The Prove Symmetric Matrixes Theorem can be proven using the properties of matrix transpose and scalar multiplication. By decomposing a matrix into its symmetric and skew symmetric parts, we can then show that the transpose of the symmetric matrix is equal to the symmetric matrix itself, and the transpose of the skew symmetric matrix is equal to the skew symmetric matrix multiplied by -1.

5. What is the significance of the Prove Symmetric Matrixes Theorem?

The Prove Symmetric Matrixes Theorem is significant because it shows that any square matrix can be broken down into two simpler matrices that have unique properties. This can be useful in solving systems of linear equations, as well as in other areas of mathematics such as eigenvalues and eigenvectors. Additionally, the theorem helps us better understand the structure of matrices and their properties.

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