# When is Newton's Method guaranteed to converge to a good solution (non-linear system)?

My knowledge of Newton's Method is partial. I am trying to understand what guarantees this method can give on the solution of systems of non-linear equations.

Specifically, I have a set of non-linear equations that are easily twice differentiable. What additional conditions do I need to fulfill in order to guarantee that Newton's Method finds a good solution? How important is the starting point? if it is important, how can I guarantee that I find a good starting point?

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The items below should help you to look up further details of Newton's Method for system of nonlinear equations.

• Q-quadratically convergent from good starting guesses if the Jacobian $J(x_*)$ is nonsingular
• Exact solution in one iteration for an affine $F$ (exact at each iteration for any finite component functions of $F$)

• Not globally convergent for many problems
• Requires $J(x_k)$ at each iteration
• Each iteration requires the solution of a system of linear equations that may be singular or ill-conditioned

References:

Notes

• For Newton's method, you would choose a tolerance and use some vector norm to test that the result is good enough.
• If you choose a bad starting value, all bets are off.
• You might also want to look into quasi-Newton methods.
• For good starting values, you want to look into the Steepest Descent method, which is used to find accurate starting approximations for the Newton-based techniques.
• As an aside, you probably also want to look at and understand "Constrained" versus "Unconstrained" methods.
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Nice summary! This is really a nice resource. – amWhy Apr 24 '13 at 1:33
Agreed, so bored I am actually looking a series solution method for a DEQ (yuck!) :-) – Amzoti Apr 24 '13 at 2:36
Thanks! One point which is still not clear to me is how do I know if I reached the global solution for a specific problem, if I can tell that a starting point was good only in retrospect? For what classes of problems is global convergence guaranteed (convergence rate is less important to me)? – Bitwise Apr 24 '13 at 2:39
@Bitwise: For Newton's method, you would choose a tolerance and use some vector norm to test that it is good enough. If you choose a bad starting value, all bets are off. You might also want to look into quasi-Newton methods. For good starting values, you want to look into Steepest Descent method, which is used to find accurate starting approximations for the Newton-based techniques. – Amzoti Apr 24 '13 at 2:50