How to compute Dottie number accurately?

Question

Dottie number is root of this equation : $cos \alpha = \alpha$, $\alpha \approx 0.73908513321516064165531208767\dots$.

I wonder how can I compute it ? I have tried to do it with an approximating formula:

$\alpha = \frac{5\pi^2}{\alpha^2 + \pi^2} - 4$

I have solved this equation and i got $\alpha \approx 0.738305\dots$. So , how can i compute it accurately ? Can i use taylor series, etc. ?

Answer 1

Taylor series of order 2 gives a simple quadratic in $\alpha$: $$\alpha=1-\alpha^2/2\implies \alpha=0.\color{red}{73}2..$$ Of order 4 gives a bi-quadratic (there's a formula to solve roots of a polynomial of degree less than 5) in $\alpha^2$: $$\alpha=1-\alpha^2/2+\alpha^4/4\implies 0.\color{red}{739}2..$$ Fairly accurate for practical purposes wherein the correct value is $0.739085...$

Answer 2

Using Newton's method,

$$\alpha = \alpha + \frac{\cos \alpha - \alpha}{\sin\alpha + 1}$$ Use this for a fixed-point iteration with chosen starting value ($\alpha_0 := \frac1{\sqrt2} = 0.7\color{red}{071}\ldots$ seems like a good choice)

Thus

$$\alpha_1 = \frac1{\sqrt 2} + \frac{\cos\frac1{\sqrt2}-\frac1{\sqrt2}}{\sin\frac1{\sqrt2} + 1} = 0.739\color{red}3\ldots \\ \alpha_2 = \frac1{\sqrt 2} + \frac{\cos\frac1{\sqrt2}-\frac1{\sqrt2}}{\sin\frac1{\sqrt2} + 1} + \frac{\cos\left(\frac1{\sqrt 2} + \frac{\cos\frac1{\sqrt2}-\frac1{\sqrt2}}{\sin\frac1{\sqrt2} + 1}\right) - \frac1{\sqrt 2} - \frac{\cos\frac1{\sqrt2}-\frac1{\sqrt2}}{\sin\frac1{\sqrt2} + 1}}{\sin\left(\frac1{\sqrt 2} + \frac{\cos\frac1{\sqrt2}-\frac1{\sqrt2}}{\sin\frac1{\sqrt2} + 1}\right) + 1} = 0.7390851\color{red}4\ldots$$

As you can see, it converges quickly. Only one more iteration gives

$$\alpha_3 = 0.73908513321516\color{red}1\ldots$$

Which is equal to $\alpha$ within the IEEE double precision standard. For $\alpha_0 = 0.7$ you need one more iteration for the same result, $0.739$ only requires two iterations and from $0.73908513$, one iteration is enough for double-precision.

Answer 3

Let $a_{0}$ be the start input (it's better to choose one close to Dottie's Number). For example, I'll choose it as $a_{0} = 0.73$.

$$a_{n} = \cos(a_{n-1})$$ $$ \lim_{n \to \infty} a_{n} = \text{Dottie's Number}$$

Let's begin the calculations: $$a_{1} = \cos(a_{0}) = \cos(0.73) = 0.745174402...$$ $$a_{2} = \cos(a_{1}) = \cos(0.745174402...) = 0.734969653...$$ $$a_{3} = \cos(a_{2}) = \cos(0.734969653...) = 0.741851103...$$ $$a_{4} = \cos(a_{3}) = \cos(0.741851103...) = 0.737219118...$$ $$\dots$$ $$a_{55} = 0,739085133...$$

Of course it is not easy to do by hand, but for a computer it's a piece of cake.

Answer 4

With $a_0=1$ and

$$a_{n+1}=\cos(a_n)$$

It must follow that

$$\alpha=\lim_{n\to\infty} a_n$$

Unfortunately, this converges extremely slowly. One can consider, more generally,

$$a(n,k,x)=\begin{cases}a(n,k-1,(a(n,k-1,x)+a(n+1,k-1,x))/2),&k>0\\a(n-1,0,\cos(x)),&n>k=0\\x,&n=k=0\end{cases}$$

From which it is easily seen that

$$\alpha=\lim_{n\to\infty}a(n,k,x)$$

For any $k\in\Bbb N$ and $x\in\Bbb R$.

A quick implementation is given: https://repl.it/LHoa/12

A few values of $a(n,k,1)$ are provided below:

   n | 0                   1                   2                   3  
k
_
0      1.0000000000000000  0.5403023058681398  0.8575532158463934  0.6542897904977791
1      0.7701511529340699  0.7655325029045684  0.7467463120692533  0.7437002334049706
2      0.7436266020461714  0.7403751852701087  0.7392909637993013  0.7391335465331998
3      0.7391414918542529  0.7390881161559403  0.7390852375353876  0.7390851380676647

α ≈ 0.7390851332151607, so $a(3,3,1)$ is accurate to 8 places. Estimating from the first few values, it appears $a(n,n,1)$ is accurate to $\alpha$ for the first $≈n^2$ places.

Answer 5

As @Lutz Lehmann wrote in comments, we have $$0=x-\cos(x)=\frac{\pi -2 \sqrt{2}}{4}+\left(1+\frac{1}{\sqrt{2}}\right)\left(x-\frac{\pi }{4}\right)+\frac{1}{\sqrt{2}}\sum_{n=2}^\infty \frac{(-1)^{\left\lfloor \frac{n+3}{2}\right\rfloor }}{n!}\left(x-\frac{\pi }{4}\right)^n$$ Truncating to some order and using series reversion $$x=\frac \pi 4+\sum_{n=1}^p \frac{a_n}{n!}\, z^n \qquad \text{with} \qquad z=\frac{2 \sqrt{2}-\pi }{4}$$

The first $a_n$ are $$\left( \begin{array}{cc} n & a_n \\ 1 & 2-\sqrt{2} \\ 2 & 14-10 \sqrt{2} \\ 3 & 300-212 \sqrt{2} \\ 4 & 10216-7224 \sqrt{2} \\ 5 & 478480-338336 \sqrt{2} \\ 6 & 28521088-20167456 \sqrt{2} \\ 7 & 2064360896-1459723584 \sqrt{2} \\ 8 & 175785969024-124299450752 \sqrt{2} \\ 9 & 17213908329728-12172071310592 \sqrt{2} \\ 10 & 1905546569787904-1347424901364224 \sqrt{2} \\ 11 & 235279360692440064-166367631418857472 \sqrt{2} \\ 12 & 32055293262298333184-22666515238694615040 \sqrt{2} \\ 13 & 4776761007035879493632-3377680100182551924736 \sqrt{2} \\ 14 & 772827653451233424957440-546471674443854294032384 \sqrt{2} \\ 15 & 134906205026964718073135104-95393092398709458563973120 \sqrt{2} \\ \end{array} \right)$$

Using only the above terms leads to an absolute error of $9.25\times 10^{-24}$

Edit

Using my favored $\color{red}{\large 1,400}$ years old approximation $$\cos(x) \simeq\frac{\pi ^2-4x^2}{\pi ^2+x^2}\qquad \text{for}\qquad -\frac \pi 2 \leq x\leq\frac \pi 2$$ solving the cubic equation $$x^3+4 x^2+\pi ^2 x-\pi ^2=0$$ $$x=-\frac{2}{3} \left(2+\sqrt{3 \pi ^2-16} \,\,\sinh \left(\frac{1}{3} \sinh ^{-1}\left(\frac{128-63 \pi ^2}{2 \left(3 \pi ^2-16\right)^{3/2}}\right)\right)\right)=\color{red}{0.7383051}84$$

Answer 6

I will use https://en.wikipedia.org/wiki/Lagrange_inversion_theorem. Did anybody use that? There are so many answers about this number.

Let $z=f(w)=\cos w-w$ and $w=a=\frac{\pi}{2}$. Since, $f'(a)=-2\neq 0$, the inverse function at $z=f(a)=-\frac{\pi}{2}$ can be expressed as
$$g(z)=a+\sum_{n=1}^{\infty}g_n\frac{(z-f(a))^n}{n!}=\frac{\pi}{2}+\sum_{n=1}^{\infty}g_n\frac{(z+\frac{\pi}{2})^n}{n!}$$ where $$g_n=\lim_{w\rightarrow a}\frac{d^{n-1}}{dw^{n-1}}\left(\frac{w-a}{f(w)-f(a)}\right)^n=\lim_{w\rightarrow \frac{\pi}{2}}\frac{d^{n-1}}{dw^{n-1}}\left(\frac{w-\frac{\pi}{2}}{\cos w-w+\frac{\pi}{2}}\right)^n$$ I was able to compute first few coefficients by hand, but then I had to use WalframAlpha. Even indexed coefficients are zero. Odd ones upto $g_9$ are:

$$g_{1}=-\frac{1}{2}, g_3=-\frac{1}{16}, g_5=-\frac{1}{16}, g_7=-\frac{43}{256}, g_{9}=-\frac{223}{256}...$$

Hence, $$g(z)=\frac{\pi}{2}-\frac{1}{2}(z+\frac{\pi}{2})-\frac{1}{96}(z+\frac{\pi}{2})^3-\frac{1}{1920}(z+\frac{\pi}{2})^5-\frac{43}{256\times 7!}(z+\frac{\pi}{2})^7-\frac{223}{256}\frac{(z+\frac{\pi}{2})^9}{9!}-...$$ Finally, Dottie number is $D=g(0)$. (Why?) And WolframAlpha computed as $D=g(0)\approx 0.739$. Unfortunately, since I am too lazy to compute $g_{11}$, the fourth-digit is false.