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Fix implementation of complex expm1. Add tests that fail with previous implementation, but pass with the current one.

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Srinivas Vasudevan
2019-12-12 01:56:54 +00:00
committed by Rasmus Munk Larsen
parent 381f8f3139
commit 88062b7fed
2 changed files with 23 additions and 1 deletions
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19
Eigen/src/Core/MathFunctions.h
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@@ -529,7 +529,24 @@ struct expm1_impl<std::complex<RealScalar> > {
EIGEN_DEVICE_FUNC static inline std::complex<RealScalar> run(
const std::complex<RealScalar>& x) {
EIGEN_STATIC_ASSERT_NON_INTEGER(RealScalar)
return std_fallback::expm1(x);
RealScalar xr = x.real();
RealScalar xi = x.imag();
// expm1(z) = exp(z) - 1
// = exp(x + i * y) - 1
// = exp(x) * (cos(y) + i * sin(y)) - 1
// = exp(x) * cos(y) - 1 + i * exp(x) * sin(y)
// Imag(expm1(z)) = exp(x) * sin(y)
// Real(expm1(z)) = exp(x) * cos(y) - 1
// = exp(x) * cos(y) - 1.
// = expm1(x) + exp(x) * (cos(y) - 1)
// = expm1(x) + exp(x) * (2 * sin(y / 2) ** 2)
RealScalar erm1 = std_fallback::expm1(xr);
RealScalar er = erm1 + RealScalar(1.);
RealScalar sin2 = sin(xi / RealScalar(2.));
sin2 = sin2 * sin2;
RealScalar s = sin(xi);
RealScalar real_part = erm1 - RealScalar(2.) * er * sin2;
return std::complex<RealScalar>(real_part, er * s);
}
};