On Thu, Feb 18, 2021 at 01:55:13PM -0800, Tobias Neumann wrote:
> Thank you. As it turns out, complex exponentiation (^ : % -> %) with a
> Complex(DoubleFloat) exponent is also broken:
>
> Good:
> (7) -> (-1.0 :: Complex(Float))^(0.5)
>
> (7) 0.2022266248_7959507324 E -20 + %i
> Type:
> Complex(Float)
>
> Broken:
> (6) -> (-1.0 :: Complex(DoubleFloat))^(0.5)
>
> (6) 6.123233995736766e-17 - %i
> Type:
> Complex(DoubleFloat)
>
That is consequence of IEEE rules about signed zero and precedence
rules:
(21) -> m1 := -1.0 :: Complex(DoubleFloat)
(21) - 1.0
Type: Complex(DoubleFloat)
(22) -> imag(m1)
(22) -0.0
Type: DoubleFloat
(23) -> m1a := complex(-1, 0)$Complex(DoubleFloat)
(23) - 1.0
Type: Complex(DoubleFloat)
(24) -> imag(m1a)
(24) 0.0
Type: DoubleFloat
(25) -> m1a^(0.5)
(25) 6.123233995736766e-17 + %i
Type: Complex(DoubleFloat)
31) -> m1b := (-1.0) ::Complex(DoubleFloat)
(31) - 1.0
Type: Complex(DoubleFloat)
(32) -> imag(m1b)
(32) 0.0
Type: DoubleFloat
Coercion has very high precendece, so your example is equvalent to
-(1::Complex(DoubleFloat))
which according to IEEE has negative imaginary part...
--
Waldek Hebisch
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