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Fix: RootN(x, n) should be equal to -Pow(x, 1/n) for odd nth roots. #86

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Merged
IsaMorphic merged 1 commit into from
Mar 19, 2026
Merged

Fix: RootN(x, n) should be equal to -Pow(x, 1/n) for odd nth roots. #86

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14 changes: 13 additions & 1 deletion QuadrupleLib.Tests/Math/PowerAndRootTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,9 @@ public void IsSqrtCorrect(double x)
[InlineData(1.0)]
[InlineData(2.1)]
[InlineData(3.676)]
[InlineData(-1.0)]
[InlineData(-2.1)]
[InlineData(-3.676)]
public void IsCbrtCorrect(double x)
{
double y = double.Cbrt(x);
Expand All @@ -52,6 +55,9 @@ public void IsCbrtCorrect(double x)
[InlineData(1.0, 3)]
[InlineData(2.1, 3)]
[InlineData(3.676, 3)]
[InlineData(-1.0, 3)]
[InlineData(-2.1, 3)]
[InlineData(-3.676, 3)]
[InlineData(1.0, 4)]
[InlineData(2.1, 4)]
[InlineData(3.676, 4)]
Expand All @@ -68,13 +74,19 @@ public void IsRootNCorrect(double x, int n)
[InlineData(1.0, 3)]
[InlineData(2.1, 3)]
[InlineData(3.676, 3)]
[InlineData(-1.0, 3)]
[InlineData(-2.1, 3)]
[InlineData(-3.676, 3)]
[InlineData(1.0, 4)]
[InlineData(2.1, 4)]
[InlineData(3.676, 4)]
public void IsRootNEqualToPow(double x, int n)
{
Float128<TAccelerator> y0 = Float128<TAccelerator>.RootN(x, n);
Float128<TAccelerator> y1 = Float128<TAccelerator>.Pow(x, Float128<TAccelerator>.One / n);
Float128<TAccelerator> y1 = Float128<TAccelerator>.Pow(
Float128<TAccelerator>.Abs(x), Float128<TAccelerator>.One / n
);
if (x < 0.0) y1 = -y1;
AssertX.NearlyEqual(y0, y1, Precision.NearestTenThousandth);
}

Expand Down
6 changes: 3 additions & 3 deletions QuadrupleLib/Modules/MathOperations.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -560,7 +560,7 @@ public static Float128<TAccelerator> Exp2(Float128<TAccelerator> y)

public static Float128<TAccelerator> Pow(Float128<TAccelerator> x, Float128<TAccelerator> y)
{
if (IsZero(x) && IsZero(y))
if (IsZero(x) && IsZero(y))
{
return _sNaN;
}
Expand All @@ -574,7 +574,7 @@ public static Float128<TAccelerator> Pow(Float128<TAccelerator> x, Float128<TAcc

return result;
}
else
else
{
return Exp(y * Log(x));
}
Expand Down Expand Up @@ -629,7 +629,7 @@ public static Float128<TAccelerator> RootN(Float128<TAccelerator> x, int n)
}
else if (x < Zero)
{
return (n & 1) == 0 ? _sNaN : -Pow(-x, n);
return (n & 1) == 0 ? _sNaN : -Pow(-x, One / n);
}
else
{
Expand Down
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