MathOverflow will be down for maintenance for approximately 3 hours, starting Monday evening (06/24/2013) at approximately 9:00 PM Eastern time (UTC-4).
show/hide this revision's text 3 added 9 characters in body

If you write the problem of finding the closest point to $x_0$ on the cone with a Lagrange multiplier, the solution must have the form $x = (\lambda A + I )^{-1}x_0$.

If you start by diagonalizing $A$, the inverse can be computed efficiently and you can search for $\lambda$ by dichotomy. The algorithm will run in $O(n^2 \log{n})$log{1/\epsilon})$

show/hide this revision's text 2 added 2 characters in body

If you write the problem of finding the closest point to $x_0$ on the cone with a Lagrange multiplier, the solution must have the form $x = (\lambda A + I )^{-1}x_0$.

If you start by diagonalizing $A$, the inverse can be computed efficiently and you can search for \lambda $\lambda$ by dichotomy. The algorithm will run in $O(n^2 \log{n})$

show/hide this revision's text 1

If you write the problem of finding the closest point to $x_0$ on the cone with a Lagrange multiplier, the solution must have the form $x = (\lambda A + I )^{-1}x_0$.

If you start by diagonalizing $A$, the inverse can be computed efficiently and you can search for \lambda by dichotomy. The algorithm will run in $O(n^2 \log{n})$