I've been trying to do this for ages. I'm worried that it's impossible, but I have heard that it can be done by hand.

As long as I can get $x$ by hand I can obviously work out its value via calculator.

Please help, I am so stuck and desperately need assistance.

Thanks in advance!

  • $\begingroup$ Graph the two functions $e^{-x}$ and $\sin(x)$ and find where they intersect (approximately). $\endgroup$ – mattos Mar 31 '20 at 4:09
  • $\begingroup$ I should add that I'm looking for an algebraic method if possible! $\endgroup$ – Jepho55 Mar 31 '20 at 4:11
  • $\begingroup$ I'd be shocked if it's doable since Wolfram can only offer approximations. Not that "proof by Wolfram" is a thing, just a heuristic. $\endgroup$ – Eevee Trainer Mar 31 '20 at 4:16
  • 2
    $\begingroup$ @EeveeTrainer interestingly, with a mild interpretation, these are close enough to analytic answers. Not sure about the first two solutions yet, but the others clearly look like $n\pi$ for all integer $n \ge 2$. $\endgroup$ – gt6989b Mar 31 '20 at 4:21
  • 1
    $\begingroup$ @gt6989b Look here $\endgroup$ – gen-ℤ ready to perish Mar 31 '20 at 5:38

First of all, that equation is highly nonlinear, as a result analytically we may not (can not) find a solution.

First Observation: Note that when $x$ becomes very large $e^{-x}$ becomes very small so $$e^{-x}-\sin x\approx0-\sin x$$ and hence the zeros of $\sin x$ are the zeros of $e^{-x}-\sin x$. This observation also ensures that solution for $e^{-x}-\sin x=0$ exists.

Second Observation: For $x<0$ $e^{-x}$ is an increasing function and dominates $\sin x$ (Which can be seen in the graph too) so there are no zeros of the function $e^{-x}-\sin x$ for $x<0$.

So what's the solution? Well, the solution can only be found by Numerical methods. One easy way to do that by plotting graphs.

graph 1

This is the graph of $e^{-x}-\sin x$ where the first observation is visualized.

graph 2

This is the graph showing the intersection of $e^{-x}$ [Blue line] and $\sin x$ [Red line].

In one word the solutions are $x=0.589,3.096,6.285$ etc.

Hope this works.

  • $\begingroup$ Would there be a way to show that over any interval before the first solution the function as a whole is increasing? $\endgroup$ – Jepho55 Mar 31 '20 at 4:26
  • $\begingroup$ You mean decreasing right? So you wanna show that $e^{-x}-\sin x$ is decreasing in $(\infty,0]$? $\endgroup$ – Sujit Bhattacharyya Mar 31 '20 at 4:29
  • $\begingroup$ Close. I want to show that it's decreasing over (-infinity, 0.589) $\endgroup$ – Jepho55 Mar 31 '20 at 4:34

You cannot find a closed form for the zeroes, but you can find asymptotic approximations.

Let $x_n$ be the zero in $(\pi n-\frac \pi 2, \pi n+\frac \pi 2)$. Write $x_n=\pi n+\varepsilon_n$ with $|\varepsilon_n|<\frac \pi 2$.

Then $$e^{-\pi n -\varepsilon_n}=\sin(\pi n+\varepsilon_n)=(-1)^n\sin(\varepsilon_n)\tag{1}$$ Taking the limit, the left-hand-side converges to $0$. So this implies that $\varepsilon_n\rightarrow 0$. Thus, using Taylor expansions in $(1)$, $$e^{-\pi n}(1+o(1))= (-1)^n \varepsilon_n(1 +o(1))$$ which means that $$\varepsilon_n=(-1)^ne^{-\pi n}(1+o(1))$$

$$\boxed{x_n=\pi n+(-1)^n e^{-\pi n} + o(e^{-\pi n})}$$ The zeroes therefore converge exponentially fast towards multiples of $\pi$.

You can keep playing that game and plug the residual back into $(1)$ to find the next term in the expression of $x_n$, but this is already converging quite fast.

enter image description here


If you build the series expansion of the rhs of $$y=e^{-x}-\sin(x)$$ around $x=n \pi$ and later series reversion forcing $y=0$, you obtain $$x_n=n\pi+\frac{1}{1+e^{(1+i) \pi n}}+\frac{1}{2 \left(1+e^{(1+i) \pi n}\right)^3}+\cdots$$

$$\left( \begin{array}{ccc} n & \text{approximation} & \text{solution} \\ 0 & 0.5625000000000000000 & 0.5885327439818610774 \\ 1 & 3.0963808805403588820 & 3.0963639324106461156 \\ 2 & 6.2850492723072431593 & 6.2850492733825865338 \\ 3 & 9.4246972547386088459 & 9.4246972547385212191 \\ 4 & 12.566374101689367670 & 12.566374101689367677 \\ 5 & 15.707963117247215942 & 15.707963117247215942 \\ 6 & 18.849555928051171524 & 18.849555928051171524 \\ 7 & 21.991148574847125823 & 21.991148574847125823 \\ 8 & 25.132741228730507464 & 25.132741228730507464 \\ 9 & 28.274333882307613598 & 28.274333882307613598 \\ 10 & 31.415926535897955096 & 31.415926535897955096 \end{array} \right)$$


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