# Rational Function approximation of tan

I wanted to approximate $$\tan(x)$$ as a rational function by capturing the zero at integer multiples of $$\pi$$ and poles at odd integer multiples of $$\pi$$. For convenience, I will be dealing with $$\tan(\frac{\pi x}{2})$$

$$\tan(\frac{\pi x}{2}) \approx \frac{x}{x^2-1}$$ Now I will shift and sum this approximation for every even integer (where the zeros are - truncated) Now I will multiply it by the constant $$\frac{2}{\pi}$$ I check the convergence away from the origin, seems to work My question is: What is it that I'm doing here? There seems to be some validity to it. It might be converging for the whole complex plane.

• What exactly do you want to know? That your function is periodic? That it converges? – LegNaiB May 15 at 21:49
• @LegNaiB Is there a name for this? It seems to be a an identity. – grdgfgr May 15 at 21:53
• Ah you mean that your approximated function is exactly the tangens function? You want to know if that is true? – LegNaiB May 15 at 21:55
• @LegNaiB I want to know if it is true, and if this has been studied before. – grdgfgr May 15 at 21:56
• Your $0.6366$ is presumably $\frac{2}{\pi}$ – Henry May 15 at 22:06

## 4 Answers

Assuming that you want a rational approximation of $$\tan(x)$$ for the range $$(2n-1)\frac \pi 2 \leq x \leq (2n+1)\frac \pi 2$$ you could notice that the function $$g(x)=\Big[x-(2n-1)\frac \pi 2\Big]\Big[(2n+1)\frac \pi 2-x\Big]\tan(x)$$ is quite nice since the left and right asymptotes have been removed.

Now, we can build around $$x=n \pi$$ the Padé approximant of $$g(x)$$. Using the simple $$[3,2]$$ would give $$\tan(x)\sim h(x)=\frac{3(n\pi-x)\Big[ \alpha +\beta (x-n\pi)^2\Big] }{\Big[x-(2n-1)\frac \pi 2\Big]\Big[(2n+1)\frac \pi 2-x\Big]\Big[\gamma+\delta (x-n\pi)^2 \Big]}$$ with $$\alpha=15 \pi ^2 \left(12-\pi ^2\right)\qquad \qquad \beta=-720+60 \pi ^2+\pi ^4$$ $$\gamma=-180 \left(12-\pi ^2\right)\qquad \qquad \delta=72 \left(10-\pi ^2\right)$$

For illustration, using $$\epsilon=\frac \pi{100}$$

$$\int_{0}^{\frac \pi 2-\epsilon} \Big[h(x)-\tan(x)\Big]^2\,dx=5.49\times 10^{-6}$$ corresponding to a relative error of $$0.025$$%. Fig. 1: Representation of the difference $$y=h(x)-\tan(x)$$ for $$n=2$$ showing that in the two-thirds of interval $$(\frac32 \pi,\frac52 \pi)$$, the absolute value of this difference is less than $$10^{-5}$$.

We can do much better.

Edit

If we use the next approximation of $$g(x)$$, that is to say its $$[5,4]$$ Padé approximant, we have $$g(x)= -\frac{\pi^2}4(n\pi-x)\frac {1+a_1(n\pi-x)^2+a_2(n\pi-x)^4 } {1+b_1(n\pi-x)^2+b_2(n\pi-x)^4 }$$ with $$a_1=-\frac{60480-5040 \pi ^2-120 \pi ^4+\pi ^6}{9 \pi ^2 \left(1680-180 \pi ^2+\pi^4\right)}\qquad \qquad a_2=\frac{604800-65520 \pi ^2+420 \pi ^4+\pi ^6}{945 \pi ^2 \left(1680-180 \pi ^2+\pi^4\right)}$$ $$b_1=-\frac{4 \left(1620-174 \pi ^2+\pi ^4\right)}{9 \left(1680-180 \pi ^2+\pi ^4\right)}\qquad \qquad b_2=\frac{1008-112 \pi ^2+\pi ^4}{63 \left(1680-180 \pi ^2+\pi ^4\right)}$$ For this case $$\int_{0}^{\frac \pi 2-\epsilon} \Big[h(x)-\tan(x)\Big]^2\,dx=8.36\times 10^{-12}$$

The plot @Jean Marie kindly added would show that, in the two-thirds of interval $$(\frac32 \pi,\frac52 \pi)$$, the absolute value of the difference is less than $$10^{-8}$$.

• @JeanMarie. Thanks a lot ! – Claude Leibovici May 16 at 13:03
• Another time don't hesitate to ask. – Jean Marie May 16 at 13:06
• @JeanMarie.Be sure that I really appreciate.Cheers :-) – Claude Leibovici May 16 at 13:27
• Thank you. One comment: This is a nice approach for approximating the function very well; however, the residue of this function at the poles is not precisely the same as the tangent function (0.636999 vs 0.636619), since pade approximant does not enforce exact match at x=1. In blue is the error ratio of my first term approximant $-\frac{4}{\pi}\frac{x}{x^2-1}$ and yours in orange i.imgur.com/4F59t4M.png. Yours is far superior over a very wide range, and mine has 20% error for the slope at x=0. – grdgfgr May 16 at 20:14
• @grdgfgr. As I wrote, what I wrote is from far away the simplest one. If I have time, I shall try to build the next and post. Cheers :-) – Claude Leibovici May 17 at 2:11

$$\pi \tan(\pi x) =2x\sum_{n=0}^{\infty} \dfrac1{(n+\frac12)^2-x^2}$$

which is similar to yours.

The reference is

Aigner, Martin; Ziegler, Günter M. (2000). Proofs from THE BOOK (Second ed.). Springer-Verlag. p. 149. ISBN 978-3-642-00855-9. Archived from the original on 2014-03-08.

A little more searching (for "partial fraction expansion of tan") led to a proof in https://en.wikipedia.org/wiki/Partial_fractions_in_complex_analysis

• "similar" but as the same time rather different... – Jean Marie May 15 at 22:21
• Other interesting proofs of your formula here or here – Jean Marie May 15 at 22:30
• @Claude Leibovici I will do it with pleasure. It will be in the afternoon. Cheers! – Jean Marie May 16 at 7:25

Note that $$\frac x{1-x^2}= \tfrac12 \left(\frac1{-1-x} + \frac1{1-x}\right).$$ Therefore your partial sums can be rewritten as $$\tfrac12 \sum_{k=0}^{2N}\left( \frac1{-(2k+1)-x} + \frac1{2k+1-x}\right) = \sum_{k=0}^{2N}\frac x{(2k+1)^2-x^2}.$$ This shows that the limit $$N\to \infty$$ exists for all $$x$$ except the odd integers since the summand is $$O(k^{-2})$$. Then see the answer of Marty and this wikipedia page, that shows some more identities of this nature.

Use the formula: $$tan(2x) = \frac{2 \times tan(x)}{1-tan^2(x)}$$:
$$tan(2 \frac{\pi}{4} x) = \frac{-2 \times tan(\frac{\pi}{4} x)}{tan^2(\frac{\pi}{4} x)-1}$$.

So if u use the approximation: $$tan(\frac{\pi x}{4}) \approx \frac{\pi x}{4}$$ (first term in Taylor series): $$tan(2 \frac{\pi}{4} x) = \frac{-2 \times (\frac{\pi}{4} x)}{(\frac{\pi}{4} x)^2-1}$$.

Close to what u have written/used. It works well for $$0 \leq x \leq 0.8$$ but doesnt work well for $$x > 0.8$$: See the plot : 