Derivation of Cholesky Decomposition

In summary, the Cholesky Decomposition for a 3x3 matrix involves finding a lower-triangular matrix, L, such that the original matrix, S, can be expressed as the product of L and its transpose, L^t. This method is used for real matrices that are symmetric and semi-definite. It is important to note that in the solution, there was a mistake in the subscripts, but it can easily be corrected.
  • #1
BobTheLawyer
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Homework Statement


Derive Cholesky Decomposition for a 3x3 matrix

Homework Equations


IN: S is Real matrix with dimensions 3x3 and is Symmetric and semi-definite
Out: L is a Real matrix with dimensions 3x3 such that
  1. S=L*L^t
  2. L is lower-triangular

The Attempt at a Solution


We learned this in class, and here is what I have in my notes. Near the end, it starts not making sense, so I think I recorded something wrong, and I also don't fully get what's going on.

upload_2016-3-13_18-6-41.png
 

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  • #2
You have miscopied the subscripts in the second last line. Replace ##L_{32}## by ##L_{33}##, ##L_{21}## by ##L_{22}## and ##L_{31}## by ##L_{32}## and then it will follow from the line above.
 
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  • #3
andrewkirk said:
You have miscopied the subscripts in the second last line. Replace ##L_{32}## by ##L_{33}##, ##L_{21}## by ##L_{22}## and ##L_{31}## by ##L_{32}## and then it will follow from the line above.
Thank you so much! You're a life saver. This got me so confused. QR decomposition just clicked for me about 5 minutes ago, and now I get this too!
 

1. What is Cholesky Decomposition?

Cholesky Decomposition is a method used in linear algebra to decompose a symmetric, positive definite matrix into the product of a lower triangular matrix and its transpose.

2. Why is Cholesky Decomposition important?

Cholesky Decomposition is important because it can be used to solve systems of linear equations efficiently and accurately, especially for large matrices. It is also used in other mathematical applications such as optimization and data analysis.

3. How is Cholesky Decomposition different from other matrix decompositions?

Cholesky Decomposition is unique in that it only works for symmetric, positive definite matrices, while other decompositions like LU decomposition and QR decomposition can work for a wider range of matrices. Additionally, Cholesky Decomposition produces a lower triangular matrix as one of its factors, while other decompositions may produce a combination of upper and lower triangular matrices.

4. What are the applications of Cholesky Decomposition?

Cholesky Decomposition is commonly used in numerical linear algebra for solving systems of linear equations, computing matrix inverses, and performing least squares regression. It is also used in machine learning algorithms, such as principal component analysis and linear discriminant analysis.

5. Is Cholesky Decomposition always possible?

No, Cholesky Decomposition is only possible for symmetric, positive definite matrices. If a matrix does not meet these criteria, then Cholesky Decomposition cannot be performed. In these cases, other matrix decompositions may be used instead.

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