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I have this equation:$D=LRL^{H}$ where D is diagonal matrix, L is lower triangular matrix, R is positive definite matrix. How can one obtain these equations from above equation?$R^{-1}=L^{H}D^{-1}L$ $R^{-1}=(D^{\frac{-1}{2}}L)^{H}(D^{\frac{-1}{2}}L)$

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    Also you need $L$ to be invertible (which is not automatically true) in order for $D^{-1}$ to exist. If it is invertible, $D$ is positive definite and $D^{-1/2}$ exists (just take the $-1/2$ power of each diagonal element).2012-04-16

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Assuming $L$ is invertible, so is $L^H$. Take the inverse of both sides of your equation, noting that the inverse of a product of invertible matrices is the product of the inverses in reverse order:

$ D^{-1} = (L R L^H)^{-1} = (L^H)^{-1} R^{-1} L^{-1}$

Now can you see how to get $L^H D^{-1} L$ on the left side?

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    @hoka: any good linear algebra book ought to do you well.2012-04-17