Imaginary part to log(2;16^3)

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Issue #965 resolved
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For some reason, log(2;16^3) evaluates to 12+9.12440459647775110183e-41j. This, of course, is not true; the real (pun not intended) answer is 12. I checked log(2;16^2) and log(2;16^4) which give the expected answers of 8 and 16 respectively. I also checked 2^(12+9.12440459647775110183e-41j) which correctly evaluates to 4096+2.59053785920993634877e-37j and 2^12 evaluates to 4096.

I am running v0.12 on windows.

Comments (7)

  1. Helder Correia repo owner

    @Hadrien Theveneau @Pol Welter I believe you guys are in a better position to clarify this one (why the precision error when complex numbers are enabled).

  2. Pol Welter

    The error already occurs for the natural logarithm (ln), on which log is based.

    This is the relevant code from cmath.cpp

    CNumber CMath::ln(const CNumber& x)
    {
        HNumber abs = CMath::abs(x).real;
        CNumber result;
        result.real = HMath::ln(abs);
        // Principal Value logarithm
        // https://en.wikipedia.org/wiki/Complex_logarithm#Definition_of_principal_value
        auto imag = HMath::arccos(x.real / abs);
        result.imag = (x.imag.isPositive() || x.imag.isZero()) ? imag : -imag;
        return result;
    }
    

    The truncation error happens either in the absolute value (here mathematically a no-op, but will actually square and then take the square root again) and the arccos, or both.

    Some options to improve things here:

    1. Check if x is real, and use a simplified logic in that case.
    2. Use auto imag = HMath::arctan(x.imag / x.real); instead. Mathematically identical, but stays clear of arccos' singularity close to 1. This will require also treating separately the case of x.real==0, but not where it is only close.

    These options are not mutually exclusive. Personally, I'd rather have rock-solid behaviour in case of real arguments, so I suggest to implement option 1 regardless of option 2.

    Unfortunately I no longer have a tool chain set-up for SpeedCrunch.

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