search for: nondiagonaliz

...d lKa) are equal the model degenerates to a different analytic form. If you were to express the corresponding system of ordinary differential equations in the matrix form (as described in Appendix 5 of Bates and Watts (1988)), it would be a Schur triangular block and the special techniques for nondiagonalizable matrices, described in section A5.2 of that appendix, must be used. The clue here is that the first two parameter values are getting very close to each other and this shows up as a singularity. The fitted model is determined by two parameters, not three.