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Can you tell me which is the best approximation for cosine/sine functions. It should also reduce the computational complexity. I've already tried the Bhaskara I's approximation.

Can you suggest me anything better?

Thanks in advance.

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  • $\begingroup$ How accurate do you want it to be, how simple (or complex), and what operations do you allow? Also, do you want it in fixed or floating point? $\endgroup$ May 11 '15 at 6:13
  • $\begingroup$ I can afford one multiplication, and an addition/subtraction. And I want it for fixed point implementaion $\endgroup$
    – phanitej
    May 11 '15 at 6:16
  • $\begingroup$ What range of values? $\endgroup$ May 11 '15 at 6:20
  • $\begingroup$ Actually i want it for simultaneous sine and cosine functions, with the input real/ imag values in between -1 and +1. I want to use these functions for discrete fourier transform $\endgroup$
    – phanitej
    May 11 '15 at 6:23
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    $\begingroup$ Though discovered independently, the following is equivalent to Bhaskara's: $$\cos\bigg(\dfrac\pi2x\bigg) \simeq \Big(1-x^2\Big) \bigg(1-\dfrac{x^2}{4.5}\bigg),$$ for $|x|\le1.~$ $\endgroup$
    – Lucian
    May 11 '15 at 12:51
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For $-\pi\le x \le \pi $ I found $$\left(\frac{315}{2}\pi^2 - \frac{15}{2\pi^2} \right)x + \frac{175}{2\pi^6}\left( \frac{\pi^2}{5}-3\right)x^3,$$ is it of any help?

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  • $\begingroup$ Ya it is of help , but i need to do a modulo operation, to wrap the x^3 back into 0<x<pi. $\endgroup$
    – phanitej
    May 11 '15 at 6:33
  • $\begingroup$ @phanitej But does it really work? I had just found it on the net, but I'm afraid it's incorrect. Though, I can't check now. $\endgroup$ May 11 '15 at 6:46
  • $\begingroup$ I suppose typo's in this formula. Try using $x=\frac \pi 2$ or $x=\pi$. Could you tell where you did find it ? $\endgroup$ May 11 '15 at 9:02
  • $\begingroup$ @ClaudeLeibovici Yeah, I checked that once but couldn't doublecheck as I'm at school, so I thought I had typed something wrong in the calculator. It was the result of an Italian graduand, confirmed by who looked like his professor. I guess you're right about the typo. $\endgroup$ May 11 '15 at 10:22
  • $\begingroup$ @phanitej I'll leave this answer here in case someone finds the typo. In the meanwhile, I'll post another answer with a correct inequality. $\endgroup$ May 11 '15 at 10:58
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Hopefully you're interested in the following double inequality, valid for $0\le x\le\pi$: $$x\left(1-\frac{x}{\pi}\right)\le\sin x\le \frac{4x}{\pi}\left(1-\frac{x}{\pi}\right) $$

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