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Old   May 12, 2011, 11:03
Default Problems with biCGSTAB
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Felipe Hernandez
Join Date: Apr 2011
Posts: 13
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Petrov is on a distinguished road
I wrote a biconjugate gradient method in C for my FEM solver, but it does not work. (I can tell because when I use a different method, the results are correct). Can you see if there is an obvious problem with the code below?
	r = addVectors(b, multiplyR(A, x0), n, -1);
	copyVector(r, rtilde, n);
	int i;
		rho_1 = dotProduct(rtilde, r, n);
			copyVector(r, p, n);
		} else {
			beta = (rho_1/rho_2) * (alpha/omega);
			double * tempA = addVectors(p, v, n, -omega);
			p = addVectors(r, tempA, n, beta);
		v = multiplyR(A, p);
		alpha = rho_1 / (dotProduct(rtilde, v, n));
		s = addVectors(r, v, n, (-1)*alpha);
		t = multiplyR(A, s);
		omega = (dotProduct(t, s, n))/dotProduct(t, t, n);
		double* temp = addVectors(x, p, n, alpha);
		x = addVectors(temp, s, n, omega);
		r = addVectors(s, t, n, -omega);
		rho_2 = rho_1;
The code you see is the "meat" of the solver. Before comes the allocation, and after comes the free-ing of memory. The input of the function is A, the matrix; b the vector, and x0 the initial guess.

multiplyR(matrix, vector) is a function that multiplies a matrix on the left with a vector on the right.

addVectors(vector A, vector B, int n, double k) returns A+kB. (The n is length of each vector).

As you can see, I'm not using a pre-conditioner.

Any help would be much appreciated!!

I have recently tried using a gauss-siedel preconditioner. The results did not improve at all, but they did change. Does that mean that I should try more sophisticated preconditioners, that there something wrong with the algorithm above, or that I should move on to gmres or some other method?

(I think I'm just going to try gaussian elimination until I feel like tackling this problem again. I've spent far too long on it!!!)

Last edited by Petrov; May 12, 2011 at 17:14.
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