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Improve accuracy of quartic_roots (#1056)

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This commit is contained in:
Nick
2023-11-27 08:52:56 -08:00
committed by GitHub
parent f7f96150df
commit e0c17f6856
2 changed files with 17 additions and 1 deletions
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2
include/boost/math/tools/quartic_roots.hpp
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@@ -135,7 +135,7 @@ std::array<Real, 4> quartic_roots(Real a, Real b, Real c, Real d, Real e) {
}
Real s = sqrt(largest_root);
// s is nonzero, because we took care of the biquadratic case.
Real v = (p + s*s + q/s)/2;
Real v = (p + largest_root + q/s)/2;
Real u = v - q/s;
// Now solve y^2 + sy + u = 0:
auto [root0, root1] = quadratic_roots(Real(1), s, u);

16
test/quartic_roots_test.cpp
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@@ -142,6 +142,21 @@ void issue_825() {
CHECK_NAN(roots[3]);
}
void issue_1055() {
double a = 1.0;
double b = -547.5045576653938;
double c = 75042.069484941996;
double d = 273.7522788326969;
double e = 0.24965766552610175;
std::array<double, 4> roots = boost::math::tools::quartic_roots<double>(a, b, c, d, e);
// This is accurate to 1e-9 on every platform *except* cygwin/g++11/c++17:
CHECK_ABSOLUTE_ERROR(-0.00182420203946279, roots[0], 1e-6);
CHECK_ABSOLUTE_ERROR(-0.00182370927680797, roots[1], 1e-6);
CHECK_NAN(roots[2]);
CHECK_NAN(roots[3]);
}
int main()
{
test_zero_coefficients<float>();
@@ -150,5 +165,6 @@ int main()
test_zero_coefficients<long double>();
#endif
issue_825();
issue_1055();
return boost::math::test::report_errors();
}