// f(x) = ax^3 + bx^2 + cx + d = 0
public static double[] Solve(double a, double b, double c, double d)
{
if (a == 0)
{
throw new ArgumentException("The value must not be 0.", nameof(a));
}
if (a < 0)
{
return(Solve(-a, -b, -c, -d));
}
var f = CreateFunction(a, b, c, d);
var xc = -b / (3 * a);
var yc = f(xc);
if (yc < 0)
{
return(Solve(a, -b, c, -d).Reverse().Select(x => - x).ToArray());
}
var f1 = CreateDerivative(a, b, c);
// 自明解
if (yc == 0 && f1(xc) >= 0)
{
return new[] { xc }
}
;
// xc より小さい実数解
var x1 = SolveNegative();
var p = a * x1 + b;
var q = p * x1 + c;
return(QuadraticEquation0.Solve(a, p, q).Prepend(x1).ToArray());
double SolveNegative()
{
var x0 = -1D;
while (f(xc + x0) > 0)
{
x0 *= 2;
}
return(NewtonMethod.Solve(f, f1, xc + x0));
}
}
}