int3d - definite 3D integral by quadrature and cubature method
The function calculates an approximation to a given vector of definite integrals
I I I (F ,F ,...,F ) dx(3)dx(2)dx(1),
1 2 numfun
where the region of integration is a collection of NUMTET tetrahedrons and where
F = F (X(1),X(2),X(3)), J = 1,2,...,NUMFUN.
J J
A globally adaptive strategy is applied in order to compute approximations result(k) hopefully satisfying, for each component of I, the following claim for accuracy: ABS(I(K)-RESULT(K))<=MAX(EPSABS,EPSREL*ABS(I(K)))
int3d repeatedly subdivides the tetrahedrons with greatest estimated errors and estimates the integrals and the errors over the new subtetrahedrons until the error request is met or MAXPTS function evaluations have been used.
A 43 point integration rule with all evaluation points inside the tetrahedron is applied. The rule has polynomial degree 8.
If the values of the input parameters EPSABS or EPSREL are selected great enough, an integration rule is applied over each tetrahedron and the results are added up to give the approximations RESULT(K) . No further subdivision of the tetrahedrons will then be applied.
When int3d computes estimates to a vector of integrals, all components of the vector are given the same treatment. That is, I(Fj) and I(Fk) for
j not equal to k , are estimated with the same subdivision of the region of integration. For integrals with enough similarity, we may save time by applying int3d to all integrands in one call. For integrals that varies continuously as functions of some parameter, the estimates produced by int3d will also vary continuously when the same subdivision is applied to all components. This will generally not be the case when the different components are given separate treatment.
On the other hand this feature should be used with caution when the different components of the integrals require clearly different subdivisions.
Fortran routine dcutet.f
X=[0;1;0;0];
Y=[0;0;1;0];
Z=[0;0;0;1];
[RESULT,ERROR]=int3d(X,Y,Z,'int3dex')
// computes the integrand exp(x*x+y*y+z*z) over the
//tetrahedron (0.,0.,0.),(1.,0.,0.),(0.,1.,0.),(0.,0.,1.)
//integration over a cube -1<=x<=1;-1<=y<=1;-1<=z<=1
// bottom -top- right -left- front -rear-
X=[ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0;
-1,-1, -1,-1, 1, 1, -1,-1, -1,-1, -1,-1;
1,-1, 1,-1, 1, 1, -1,-1, 1,-1, 1,-1;
1, 1, 1, 1, 1, 1, -1,-1, 1, 1, 1, 1];
Y=[ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0;
-1,-1, -1,-1, -1, 1, -1, 1, -1,-1, 1, 1;
-1, 1, -1, 1, 1, 1, 1, 1, -1,-1, 1, 1;
1, 1, 1, 1, -1,-1, -1,-1, -1,-1, 1, 1];
Z=[ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0;
-1,-1, 1, 1, -1, 1, -1, 1, -1,-1, -1,-1;
-1,-1, 1, 1, -1,-1, -1,-1, -1, 1, -1, 1;
-1,-1, 1, 1, 1,-1, 1,-1, 1, 1, 1, 1];
function v=f(xyz,numfun),v=exp(xyz'*xyz),endfunction
[result,err]=int3d(X,Y,Z,f,1,[0,100000,1.d-5,1.d-7])
function v=f(xyz,numfun),v=1,endfunction
[result,err]=int3d(X,Y,Z,f,1,[0,100000,1.d-5,1.d-7])