The other kind of problem is where the solutions are defined on a fixed form, like S-systems which is currently the most popular form for identification. The S-system formalism (Savageau, 1976; Voit, 2000) is based on approximating kinetic laws with multivariate power-law functions. A model consists of n non-linear ODEs and the generic form of equation i reads (1)  where X is a vector (length n) of dependent variables, α and β are vectors (length n) of non-negative rate constants and g and h are matrices (n × n) of kinetic orders, that can be negative as well as positive. The structure of the system is defined by the non-zero elements of g and h, and for realistic models most of these parameters are zero.