Solving system with sparse matrices

Hello, I´m developing a code in which I need to do the follow operation: r=K\F, where r is a vector of displacements, K the stiffness matrix and F a vector of forces. Since the system is very large, I have used the sparse function for K and F (many of their elements are zero). I expected to obtain similar results doing: r=K\F; or K=sparse(K); F=sparse(F); r=K\F; However, I don´t obtain the same results (indeed the variation is very remarkable). Does anyone know if the there are limitations for using the sparse function? Thanks in advanced.

Carlos

[Torsten]

Is K regular and square ?

Test the reliability of the results obtained in both cases by calculating K*r-F.

Best wishes

Torsten.