# Methods to approximate $f(x) = \sin(x + f(x))$

This question has kind of two parts:

1. Is it possible to find a closed non recursive form of $f(x)$ where $f(x) = \sin(x + f(x))$.

2. If the answer to 1 is no, then is it possible to approximate this function in a less computationally expensive way than $f_k(x) = \sin(x + f_{k-1}(x))$

Using the $f_k(x)$ at a depth of $k>10000$ I seem to be able to find that $f_k(x)$ as $k$ approaches ∞ converges to the following graph, where the blue line is $f_{10000}(x)$ and the orange line is $\sin(x)$.

I have tried searching google and here but I can't seem to find any information on what techniques I could use to find a non-recursive form or a better approximation of an infinitely recursive function like $f(x)$.

• There's Fourier series. See the answer to this math.stackexchange.com/q/1652612/269624 – Yuriy S Feb 10 '18 at 20:14
• However the Fourier series contain Bessel functions. So they might not offer any advantage when it comes to computation. Though they are much better to work with analytically – Yuriy S Feb 10 '18 at 20:58

[More of an extended comment than an actual answer.]

This can sort of be done in closed form. By differentiating $f(x) = \sin(x+f(x))$ twice, one obtains a differential equation $$f''(x)=\frac{f'(x) f''(x)}{f'(x)+1}-f(x) \left(f'(x)+1\right)^2$$ subject to $f(0) = 0$, which Mathematica tells me is solved by letting $f$ be an inverse of the function $$y \mapsto \pm \tan^{-1}\left(\frac{y\sqrt{1+2a-y^2}}{y^2-2a-1}\right) - y$$ for some constant $a$. It should be possible to fix the value of $a$ using another value of $f$ (perhaps at $x=\pi$); note, however, that your function looks very likely to be discontinuous, so it may be necessary to stitch together pieces of the solution on different intervals; and the boundary condition I applied, namely $f(0) = 0$, does not constrain the solution if we move past a discontinuity.

You can efficiently approximate $f(c)$ by solving $\sin(x + c) - x = 0$ for $x$. This always has a unique solution, the solution always lies in $[-1, 1]$ and the function is always decreasing. Therefore we can simply use binary search to get one fractional bit of precision per iteration.

E.g. in Python:

import math
def f(x, prec=1e-10):
lo = -1.1
hi = 1.1
while hi - lo > prec:
mid = (lo + hi) / 2
if math.sin(x + mid) - mid > 0:
lo = mid
else:
hi = mid
return (lo + hi) / 2


And to show the graph:

import matplotlib.pyplot as plt
import numpy as np
x = np.linspace(-2*math.pi, 2*math.pi, 1000)
y = list(map(f, x))
plt.grid(True)
plt.plot(x, y)
plt.show()


Yes, there is a non recursive form as a power series in $\;y=x^{1/3}.\;$ We have the result that $$f(x) = 6^{1/3}y^1 -\frac{9}{10}y^3 + \frac{3}{350}(9/2)^{1/3}y^5 + \frac1{350}(3/4)^{1/3}y^7 + \frac{1161}{2156000}y^9 + O(y^{11}) .$$ Of course, this is only the power series around $0$. The function $f(x)$ is an odd function with period $2\pi$ just like $\sin(x).$ You can use Newton's method to approximate the function to arbitrary accuracy given an initial approximation value.

Following the idea of the link in @Yuriy S's comment, notice that the graph of $y = f(x)$ can be parametrized by

$$(x, y) = (t - \sin t, \sin t). \tag{1}$$

This already provides one way of computing $y = f(x)$. You can solve $x = t - \sin t$ in terms of $t$ and compute $y = \sin(t)$. You may utilize various methods to numerically solve this, for instance, Newton's method will work.

Another consequence of $\text{(1)}$ is that we obtain the following Fourier series

$$f(x) = \sum_{n=1}^{\infty} \frac{2J_n(n)}{n} \sin (nx).$$

This series converges only polynomially fast (and hence computationally inefficient), however, as we have the asymptotics

$$\frac{2J_n(n)}{n} \sim \frac{1}{\Gamma(\frac{2}{3})} \left( \frac{2}{\sqrt{3}\,n} \right)^{4/3} \quad \text{as } n\to\infty.$$

Alternatively, if $\langle x \rangle = x \text{ mod } 1$ denotes the fractional part of $x$, then

$$f(x) = \int_{0}^{\pi} \left( \left< \frac{t - \sin t - x}{2\pi} \right> - \left< \frac{t - \sin t + x}{2\pi} \right> \right) \, dt.$$

Due the the jump discontinuity, this formula is not terribly nice. Indeed it requires the knowledge of jumps of the integrand, which boils down to solving the equation $x = t - \sin t$. Not to mention, it is simply easier to work with $\text{(1)}$.