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 Sep 24 awarded Autobiographer Nov 17 comment Coordinates of point that is on the end of perpendicular line This question appears to be off-topic because it is about someone's geometry homework. Nov 28 awarded Supporter Sep 15 comment Is there a faster way to calculate a few diagonal elements of the inverse of a huge symmetric positive definite matrix? @Gottried Helms: thank you and apologies for my initial comment yesterday. I manage to not only make a mental mistake in the answer I wrote, but also a programming mistake which convinced me I was right when I was in error. Sep 14 comment Is there a faster way to calculate a few diagonal elements of the inverse of a huge symmetric positive definite matrix? Not quite. The incomplete factorization also gives $R_{10}$, but $R_{11}$ is also required to solve for $B_{00}$. That implies the full factorization is necessary. Sep 14 revised Is there a faster way to calculate a few diagonal elements of the inverse of a huge symmetric positive definite matrix? added 28 characters in body Sep 14 revised Is there a faster way to calculate a few diagonal elements of the inverse of a huge symmetric positive definite matrix? Added a corrected solution Sep 14 comment Is there a faster way to calculate a few diagonal elements of the inverse of a huge symmetric positive definite matrix? @Jonas Heidelberg: I am terribly sorry, but I think I may have given you an incorrect solution to your problem. I hope it didn't waster too much of your time. Sep 14 revised Is there a faster way to calculate a few diagonal elements of the inverse of a huge symmetric positive definite matrix? Original solution probably incorrect. Sep 14 revised Is there a faster way to calculate a few diagonal elements of the inverse of a huge symmetric positive definite matrix? Tidied up the last section a little bit. Sep 14 awarded Editor Sep 14 revised Is there a faster way to calculate a few diagonal elements of the inverse of a huge symmetric positive definite matrix? Added more detail to the explanation. Sep 14 awarded Teacher Sep 14 answered Is there a faster way to calculate a few diagonal elements of the inverse of a huge symmetric positive definite matrix?