## Explicit formula for Cholesky factorization in a special case

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I have a positive definite matrix of the form $Q+sI-\alpha J$ ($s>2, 0 < \alpha <1$ and $J$ is the all-ones matrix), where $Q$ is "nice", nonnegative and known. I'd like to know if there is a way to obtain an explicit expression for the Cholesky factorization of my matrix in this special case. Thanks!

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## 1 Answer

The matrix $\alpha J$ is a rank one matrix, so there are simple update/downdate formulas for computing the Choleksy factorization of $Q+sI-\alpha J$ if you start with the factorization of $Q+sI$.

I'm not aware of any update formulas that get you from the Cholesky factorization of $Q$ to a Cholesky factorization of $Q+sI$.

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Thanks! Can you give me a particular reference? My focus is on theory, not computation. – Felix Goldberg Apr 25 2012 at 7:53
A classic reference (and the paper is available online as a free .pdf) is:  P. Gill, G. Golub, W. Murray, and M. Saunders. Methods for modifying matrix factorizations. Mathematics of Computation, 126(28):505-535, 1974.  stanford.edu/group/SOL/papers/ggms74.pdf – Brian Borchers Apr 26 2012 at 1:02
Let me also mention that if $\alpha$ is fixed and you want to vary $s$, then you might find that a better way to go is to compute the eigenvalue decomposition of $Q-\alpha J$, and then adjust for $sI$ by adding $s$ to the eigenvalues.   – Brian Borchers Apr 26 2012 at 4:52
Well, actually it's $\alpha$ that's varying and $s$ is fixed. Thanks for the reference, I'll be sure to read. – Felix Goldberg Apr 29 2012 at 14:08
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