A Multi-Parameter Singular Perturbation Analysis of the Robertson Model

The Robertson model describing a chemical reaction involving three reactants is one of the classical examples of stiffness in ODEs. The stiffness is caused by the occurrence of three reaction rates $k_1,k_2$, and $k_3$, with largely differing orders of magnitude, acting as parameters. The model has been widely used as a numerical test problem. Surprisingly, no asymptotic analysis of this multiscale problem seems to exist. In this paper we provide a full asymptotic analysis of the Robertson model under the assumption $k_1,k_3\ll k_2$. We rewrite the equations as a two-parameter singular perturbation problem in the rescaled small parameters $({\epsilon }_1,{\epsilon }_2):=(k_1\mathrm{/}{k}_2,k_3\mathrm{/}{k}_2)$, which we then analyze using geometric singular perturbation theory (GSPT). To deal with the multi-parameter singular structure, we perform blow-ups in parameter- and variable space. We identify four distinct regimes in a neighbourhood of the singular limit $({\epsilon }_1,{\epsilon }_2)=(0,0)$. Within these four regimes we use GSPT and additional blow-ups to analyze the dynamics and the structure of solutions. Our asymptotic results are in excellent qualitative and quantitative agreement with the numerics.