Questions tagged [newton-raphson]

This tag is for questions regarding the Newton–Raphson method. In numerical analysis the Newton–Raphson method is a method for finding successively better approximations to the roots (or zeroes) of a real-valued function.

The Newton–Raphson method in one variable is implemented as follows:

The method starts with a function $f$ defined over the real numbers, the function's derivative $f'$, and an initial guess $x_0$ for a root of the function $f$. If the function satisfies the assumptions made in the derivation of the formula and the initial guess is close, then a better approximation $x_1$ is $$x_1 = x_0 -\frac{f(x_0)}{f'(x_0)}$$

The process is repeated as

$$x_{n+1} = x_n -\frac{f(x_n)}{f'_n(x_n)}$$

until a sufficiently accurate value is reached.

841 questions
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Why is it not possible to generate an explicit formula for Newton's method?

Going through the recursive formula for approximating roots every time is extraordinarily tedious, so I was wondering why there was no formula that computed the $n$th iteration of Newton's method.
15
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1 answer

Bounding the basins of attraction of Newton's method

In general, Newton's method for root finding has a "bubbly" boundary between basins of convergence for different roots. This is where fractals are usually created from. But outside these "bubbly" boundaries there are very clear areas where there's…
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2 answers

Pattern of Newton-Raphson iteration $x\mapsto\frac{1}{2}(x+\frac{q}{x})$ over finite fields

While playing with Newton-Raphson method over finite field $\mathbb{F}_p$, I noticed some cute patterns that I can't explain out of my brain contaminated with analysis. Here is the setting: Setting. Let $p$ be an odd prime and $q \in \mathbb{F}_p$.…
Sangchul Lee
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Convergence of the quadratic map $\left(x-\left(x-\left(x- \dots \right)^2 \right)^2 \right)^2$?

Edit - I changed the title and much of the body to better reflect my full question. The old one I don't really care about, although I appreciate Fabian's answer of course. Here is the plot for the function (or map), defined as…
12
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4 answers

Newton's method intuition

In optimisation the Newton step is $-\nabla^2f(x)^{-1}\nabla f(x)$. Could someone offer an intuitive explanation of why the Newton direction is a good search direction? For example I can think of steepest gradient decent as a ball rolling down a…
11
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Newton's method for square roots 'jumps' through the continued fraction convergents

I know that Newton's method approximately doubles the number of the correct digits on each step, but I noticed that it also doubles the number of terms in the continued fraction, at least for square roots. Explanation. If we start Newton's…
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Convergence of Newton Method for monotonic polynomials

Consider a polynomial $p : \mathbb R \to \mathbb R$ with $p'(x) > 0$ for all $x \in \mathbb R$. The function $p$ has exactly one real zero. Will the Newton method $$x_{n+1} = x_n - \frac{p(x_n)}{p'(x_n)}$$ converge for all $x_0 \in \mathbb…
flawr
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does this Newton-like iterative root finding method based on the hyperbolic tangent function have a name?

I've recently discovered that modifying the standard Newton-Raphson iteration by "squashing" $\frac{f (t)}{\dot{f} (t)}$ with the hyperbolic tangent function so that the iteration function is $$N_f (t) = t - \tanh \left( \frac{f (t)}{\dot{f} (t)}…
9
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2 answers

When does Newton-Raphson Converge/Diverge?

Is there an analytical way to know an interval where all points when used in Newton-Raphson will converge/diverge? I am aware that Newton-Raphson is a special case of fixed point iteration, where: $$ g(x) = x - \frac{f(x)}{f'(x)} $$ Also I've read…
9
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4 answers

Solving $\ln{x}=\tan{x}$ with infinitely many solutions

Lets take $f(x)=\ln{x}$ and $g(x)=\tan{x}$ When $f(x)=g(x)$ that is $\ln{x}=\tan{x}$, we see that the graph is like: Hence we see that there are infinitely many solutions to $x$ but the two graphs do not coincide (like while solving…
9
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4 answers

How to solve $x^2 = e^x$

The question is to find $x$ in: \begin{equation*} x^2=e^x \end{equation*} I know Newton's method and hence could find the approx as $x\approx -0.7034674225$ from \begin{equation*} x_{n+1} = x_n -…
9
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1 answer

Reference request for Newton's method on a convex/concave function

The proof featured at http://planetmath.org/newtonsmethodworksforconvexrealfunctions shows that, roughly speaking, Newton's method is unconditionally convergent for convex/concave functions. I am looking for a textbook or paper reference of this…
parsiad
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Why does multiplying by the inverse of the Jacobian matrix generalize the Newton-Raphson method to n dimensions?

I don't fully understand how multiplying by the inverse of the Jacobian matrix generalizes the Newton-Raphson method to $n$ dimensions. In other words, I don't understand how this: $x_{n+1} = x_n - \frac{f(x_n)}{f'(x_n)}$ is equivalent to this in 2…
Autechre
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8
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Prove or disprove - Newton's method convergence in higher dimensions

It's not an exercise for uni or anything like that, just something that's been bothering me a bit and I can't seem to find useful information on the web on the matter. When talking about real valued scalar functions, we know that newton's method…
7
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1 answer

How to estimate distance from root to nearest immediate basin boundary for Newton's method in one complex variable?

Context: I want to check that the atom domain size estimate is smaller than the inradius of the Newton immediate basin, for centers of hyperbolic components in the Mandelbrot set, and thus justify using Newton's method to find the center given an…
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