# When does the Newton Raphson method fail?

Can someone please tell me the conditions under which the Newton Raphson method will not converge?
I looked around online, and couldn't find a general way to determine.
For example, for the Fixed Point iteration method, there is a simple way of determining: if we have $g(x_{n})=x_{n+1}$, then $|g'(x)|<1$ implies that the series $g$ will converge to its fixed point, but in the Newton Raphson method, It seems like it is totally depends on "luck", meaning if you were lucky enough to pick a "good" initial guess or not.

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The condition for the fixed iteration method to converge is far from that"simple. And if you look closely, Newton-Raphson is fixed-point iteration, just of a different function. – vonbrand Mar 25 '14 at 16:54
Some examples are given by lhf in answering this question – Ross Millikan Mar 25 '14 at 16:55

Since the NR method can be written as follows: $$x_{n+1}=x_n-\frac{f(x_n)}{f'(x_n)},$$ it means that it cannot converge as soon as:

• $x_n$ is a local minimum/maximum of $f(\cdot)$;

• $f(\cdot)$ has a multiple root, i.e. with multiplicity greater than 1.

Hope it helps.

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Hi, @7raiden7, thanks for the help. Although these are good points, and helped a lot, this still will not help me figure out in advance which $x_0$'s will cause convergence. for example, according to your first point, I can find myself picking some $x_0$, and on sheer luck (or, more accurately, bad luck) "step on a mine" - meaning stepping on an $x_k$ which satisfies $f'(x_k)=0$. and that's exactly what I wand to avoid. (the second point can help me decide to avoid the NR method altogether). – so.very.tired Mar 25 '14 at 17:14
Therefore you may want to check some modified NR method. I've never gone through that, but I guess that for you should pose a tolerance for $f'(x_n)$, like a flux limiter. Unfortunately this is a very tough field, it is very far from being simple. I recommend you to read some paper about fixed point theory/numerical root finding. You should also consider reducing the accuracy in favour of more stability, i.e. going for the bisection method or affine ones. – 7raiden7 Mar 25 '14 at 17:18

Think geometrically about how the method works. We draw a tangent line to a curve. We follow that tangent down (or up) to the $x$-axis. Then, we jump up to the function at that point and repeat.

Now, what happens if the tangent line overshoots the root and sends us to a point on the function where the tangent line has the opposite slope? Can you visualize the ping-pong behavior?

What happens if the slope is very small (i.e. a flat tangent line)? What happens if the slope is very steep (i.e. a nearly vertical tangent line)?

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yes, I understand the geometry implication. that alone made me wonder about the unexpected behavior of an initial $x_0$. – so.very.tired Mar 25 '14 at 17:07
In fact, some several trips around the sun ago, I recall this exact example being on my numerical analysis final exam. – Arkamis Mar 25 '14 at 17:21