# Numerical Integration

(Difference between revisions)
 Revision as of 11:33, 3 November 2009 (view source)JMora (Talk | contribs) (→Gauss-Legendre Numerical Integration)← Older edit Revision as of 11:33, 3 November 2009 (view source)JMora (Talk | contribs) (→Gauss-Legendre Numerical Integration)Newer edit → Line 27: Line 27: − {| border="1" cellpadding="5" cellspacing="0" class="wikitable" style="margin:auto; background:white;" + {| border="1" cellpadding="1" cellspacing="0" class="wikitable" style="margin:auto; background:white;" ! Number of points, ''p'' !! Points, ''ξ''''i'' !! Weights, ''w''''i'' ! Number of points, ''p'' !! Points, ''ξ''''i'' !! Weights, ''w''''i'' |- align="center" |- align="center"

## Revision as of 11:33, 3 November 2009

Numerical integration refers to all the procedures, algorithms and techniques in the numerical analysis to obtain an approximate solution to a definite integral.

That is, how to obtain a numerical value of: $\int_{\lambda_a}^{\lambda_b}\! f(\lambda)\, d\lambda.$

where $\lambda \,$ can be a 1D, 2D or 3D domain.

For our interest in the Finite Element Method, the purpose is to describe how the element matrices can be integrated numerically.

## Gauss-Legendre Numerical Integration

To fix the most basic concepts on Numerical Integration, we will focus our description on a one dimensional integration using the Gauss-Legendre quadrature, that is, to solve: $I=\int_{-1}^{+1} f(\xi) d\xi$

The Gauss-Legendre quadrature establish that the definite integral of a function can be approximate by using a weighted sum of function values at specified points within the domain of integration. An p-point Gaussian quadrature rule is constructed to yield an exact result for polynomials of degree 2p − 1 or less by a suitable choice of the points $\xi_i \,$   and weights $w_i \,$   for $i = 1, \cdots p \,$. $\int_{-1}^{+1} f(\xi)\,d\xi \approx \sum_{i=1}^p w_i f(\xi_i)$

The coordinates and related weights are:

Number of points, p Points, ξi Weights, wi
1 0 2
2 $\pm\sqrt{1/3}$ 1
3 0 89 $\pm\sqrt{3/5}$ 59
4 $\pm\sqrt{\Big( 3 - 2\sqrt{6/5} \Big)/7}$ $\tfrac{18+\sqrt{30}}{36}$ $\pm\sqrt{\Big( 3 + 2\sqrt{6/5} \Big)/7}$ $\tfrac{18-\sqrt{30}}{36}$
5 0 128225 $\pm\tfrac13\sqrt{5-2\sqrt{10/7}}$ $\tfrac{322+13\sqrt{70}}{900}$ $\pm\tfrac13\sqrt{5+2\sqrt{10/7}}$ $\tfrac{322-13\sqrt{70}}{900}$