The following double precision numerical integration or quadrature rules are supported in Math.NET Numerics under the `MathNet.Numerics.Integration` namespace. Unless stated otherwise, the examples below evaluate the integral $\int_0^{10} x^2 \, dx = \frac{1000}{3} \approx 333.\overline{3}$.
//Approximate value of the integral using IntegrateComposite with 15 partitions is: 334.074074074074
Console.WriteLine("Approximate value using IntegrateComposite with 15 partitions is: " + composite);
// Two point approximation
double twoPoint = NewtonCotesTrapeziumRule.IntegrateTwoPoint(x => x * x, 0.0, 10.0);
//Approximate value using IntegrateTwoPoint is: 500
Console.WriteLine("Approximate value using IntegrateTwoPoint is: " + twoPoint);
Double-Exponential Transformation
---------------------------------
The Double-Exponential Transformation is suited for integration of smooth functions with no discontinuities, derivative discontinuities, and poles inside the interval.
// Approximate value using a relative error of 1e-5 is: 333.333333333332
Console.WriteLine("Approximate value using a relative error of 1e-5 is: " + integrate);
Gauss-Legendre Rule
-------------------
A fixed-order Gauss-Legendre integration routine is provided for fast integration of smooth functions with known polynomial order. The N-point Gauss-Legendre rule is exact for polynomials of order $2N-1$ or less. For example, these rules are useful when integrating basis functions to form mass matrices for the Galerkin method [[GSL]](https://www.gnu.org/software/gsl/).
The basic idea of Gauss-Legendre integration is to approximate the integral of a function $f(x)$ using $N$ Weights $w_i$ and abscissas (or nodes) $x_i$.
This algorithm calculates the abscissas and weights for a given order and integration interval. For efficiency, pre-computed abscissas and weights for the orders $ N = 2 - 20, \, 32, \, 64, \, 96, 100, \, 128, \, 256, \, 512, \, 1024$ are used. Otherwise, they are calculated on the fly using Newton's method. For more information on the algorithm see [[Holoborodko, Pavel] ](http://www.holoborodko.com/pavel/numerical-methods/numerical-integration/).
### Abscissas and Weights
We'll first use the abscissas and weights to approximate an integral using a 5-point Gauss-Legendre rule
[lang=csharp]
// Create a 5-point Gauss-Legendre rule over the integration interval [0, 10]
GaussLegendreRule rule = new GaussLegendreRule(0.0, 10.0, 5);
double sum = 0; // Will hold the approximate value of the integral
for (int i = 0; i <rule.Order;i++)//rule.Order =5
{
// Access the ith abscissa and weight
sum += rule.GetWeight(i) * rule.GetAbscissa(i) * rule.GetAbscissa(i);
}
// Approximate value is: 333.333333333333
Console.WriteLine("Approximate value is: " + sum);
If you prefer direct access to the abscissas and weights, as opposed to using the methods
- ```double GetAbscissa(int i)```
- ```double GetWeight(int i)```
then use the properties `Abscissas` and `Weights`
[lang=csharp]
// Create a 5-point Gauss-Legendre rule over the integration interval [0, 10]
GaussLegendreRule rule = new GaussLegendreRule(0.0, 10.0, 5);
double[] x = rule.Abscissas; // Creates a clone and returns array of abscissas
double[] w = rule.Weights; // Creates a clone and returns array of weights
double sum = 0; // Will hold the approximate value of the integral
for (int i = 0; i <rule.Order;i++)//rule.Order =5
{
// Access the ith abscissa and weight
sum += w[i] * x[i] * x[i];
}
// Approximate value is: 333.333333333333
Console.WriteLine("Approximate value is: " + sum);;
In addition to obtaining the abscissas and weights, the order and integration interval can be obtained
[lang=csharp]
// Create a 5-point Gauss-Legendre rule over the integration interval [0, 10]
GaussLegendreRule rule = new GaussLegendreRule(0.0, 10.0, 5);
// The order of the rule is: 5
Console.WriteLine("The order of the rule is: " + rule.Order);
// The lower integral bound is 0
Console.WriteLine("The lower integral bound is: " + rule.IntervalBegin);
// The upper integral bound is 10
Console.WriteLine("The upper integral bound is: " + rule.IntervalEnd);
### Integrate Method
For convenience, we provide an overloaded static method `double Integrate(...)` which preforms 1D and 2D integration of a function. The first parameter to the method is a delegate of type `Func<double, double>` or `Func<double, double, double>` for 1D and 2D integration respectively. So for example
[lang=csharp]
// 1D integration using a 5-point Gauss-Legendre rule over the integration interval [0, 10]
Larz White
10 years ago