find roots of cubic polynomial

Question

I'm trying to find the roots of a cubic polynomial ax^3+bx^2+cx+d=0 using math.numerics. The package is great, but I'm struggling to get started with it. Please can someone explain how to find the roots, and a simple explanation on how to run the sample package from Github?

I've added a reference to the package

using MathNet.Numerics;

and this is what I've tried:

var roots = FindRoots.Cubic(d, c, b, a);
double root1=roots.item1;
double root2=roots.item2;
double root3=roots.item3;

but I get an error "The type 'Complex' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Numerics'". Adding using System.Numerics gives an error and doesn't solve the problem.

Any suggestions please?

you need to input the code in the top most part Using System.Numerics — Albert Laure, Jul 15 '16 at 03:22
that's where I put it, along with all the other assemblies, directly below ``using MathNet.Numerics``. Intellisense says the assembly is not required. — Zeus, Jul 15 '16 at 03:35

Michael Liu · Accepted Answer · 2016-07-15T03:51:35.810

5

If you're using Visual Studio, you need to right-click the References folder for your project in Solution Explorer, click Add Reference, and then select System.Numerics from the Assemblies > Framework list:

Because MathNet.Numerics.FindRoots.Cubic returns roots as complex numbers, you must use the System.Numerics.Complex type instead of double to store your roots:

using System.Numerics;
using MathNet.Numerics;

class Program
{
    static void Main()
    {
        double d = 0, c = -1, b = 0, a = 1; // x^3 - x
        var roots = FindRoots.Cubic(d, c, b, a);
        Complex root1 = roots.Item1;
        Complex root2 = roots.Item2;
        Complex root3 = roots.Item3;
    }
}

If you only want to deal with real numbers, call MathNet.Numerics.RootFinding.Cubic.RealRoots instead (which will return any complex-valued roots as Double.NaN):

using MathNet.Numerics.RootFinding;
...
var roots = Cubic.RealRoots(d, c, b); // "a" is assumed to be 1
double root1 = roots.Item1;
double root2 = roots.Item2;
double roo13 = roots.Item3;

edited Jul 15 '16 at 03:51

answered Jul 15 '16 at 03:20

Michael Liu

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Yes, he tried that, but "it gave an error". *rolleyes* – Blorgbeard Jul 15 '16 at 03:21
@Blorgbeard: The OP stated that he added `using System.Numerics;` to his code file, instead of adding a reference to the project. – Michael Liu Jul 15 '16 at 03:22
"Adding using System.Numerics gives an error and doesn't solve the problem." – Blorgbeard Jul 15 '16 at 03:22
Oh, beg your pardon, just read that again, you're right - lol. – Blorgbeard Jul 15 '16 at 03:22
I've added the reference as Michael suggested and the compile error has gone. I was just adding the ``using ...`` line to the top of the class - sorry I should have been clearer. Thanks for the suggestion. I'll also try using complex instead of double. – Zeus Jul 15 '16 at 04:21
thanks,it works! Can you explain how can I use the sample package from GitHub? I can't compile and run it. – Zeus Jul 15 '16 at 04:26
@Zeus: What happens when you try to compile and run it? – Michael Liu Jul 15 '16 at 13:37
@Michael - if I open ``Examples.sln`` and click ``Start`` in debug or release mode I get an error saying ``"A project with an Output Type of Class Library cannot be started directly"``. I'm doing something wrong, but this is new to me so I'm not sure what I should be doing. – Zeus Jul 18 '16 at 06:28
@Michael - do you know where I can download a manual on the package's various functions? – Zeus Jul 18 '16 at 06:29
See http://stackoverflow.com/questions/23996140/math-net-numerics-how-to-run-examples and http://numerics.mathdotnet.com/api/ – Michael Liu Jul 19 '16 at 02:57
Needs reference to MathNet.Numerics – Sith2021 Aug 31 '23 at 22:24

find roots of cubic polynomial

1 Answers1