## Visit to Aachen

In the past few days I was in Aachen. I had only been there once before, which was for a job interview. It was just before Christmas. I only remember that it was dark, that there was a big Christmas market which I had trouble navigating through and that the building of the RWTH where the job was was large and imposing. This time the visit was more relaxed. I was there for a small meeting organized by Sebastian Walcher centred around the topic of singular perturbation theory and with an emphasis on models coming from biology.

I visited the cathedral of which a major part is older than all comparable buildings in northern Europe (eighth century). Even for someone with such rudimentary knowledge of architecture as myself it did appear very different from other cathedrals, with an oriental look. If I had been asked beforehand to imagine what the church of Charlemagne might look like I would never have guessed anything like the reality.

I gave a talk about my work with Juliette Hell about the existence of periodic solutions of the MAP kinase cascade. The emphasis was not only on the proof of this one result but also on explaining some general techniques with the help of this particular example. A theme running through this meeting was how, given a system of ODE depending on parameters, it is possible to find a small parameter $\epsilon$ such that the limit $\epsilon\to 0$, while singular, nevertheless has some nice properties. In particular the aim is to obtain a limiting system with less equations and less parameters. Then we would like to lift some features of the dynamics of the limiting system to the full system. One talk, which I would like to mention, was by Satya Samal of the Joint Research Centre for Computational Biomedicine on tropical geometry. This is a term which I have often heard without understanding what it means intrinsically or how it can be applied to the study of chemical reaction networks. A lot of what was in the introduction of the talk can also be found in a paper by Radulescu et al. (Math. Modelling of Natural Phenomena, 10, 124). There was one thing in the talk which I found enlightening, which is not in the paper and which I have not seen elsewhere. I will try to explain it now. In this tropical business an algebra is introduced with ‘funny’ definitions of addition and multiplication for real numbers. It seems that often the formal properties of these operations are explained and then the calculations start. This is perhaps satisfactory for people with a strong affinity for algebra. I would like something more, an intuitive explanation of where these things come from. Let us consider multiplication first. If I start with two real numbers $a$ and $b$, multiply them with $\log\epsilon$ and exponentiate, giving the powers of $\epsilon$ with these exponents, then multiply the results and finally take the logarithm and divide by $\log\epsilon$ I get the sum $a+b$. This should explain why the tropical equivalent of the product is the sum. Now let us consider the sum. I start with numbers $a$ and $b$, take powers of a quantity $\epsilon$ with these exponents as before, then sum the results and take the logarithm. Thus we get $\log (\epsilon^a+\epsilon^b)$. Now we need an extra step, namely to take the limit $\epsilon\to 0$. If $b>a$ then the second term is negligible with respect to the first for $\epsilon$ small and the limit is $a\log\epsilon$. If $a>b$ then the limit is $b\log\epsilon$. Thus after dividing by $\log\epsilon$ I get $\min\{a,b\}$. This quantity is the tropical equivalent of the sum. I expect that this discussion might need some improvement but it does show on some level where the ‘funny’ operations come from. It also has the following intuitive interpretation related to asymptotic expansions. If I have two functions whose leading order contributions in the limit $\epsilon\to 0$ are given by the powers $a$ and $b$ then the leading order of the product is the power given by the tropical product of $a$ and $b$ while the leading order of the sum is the power given by the tropical sum of $a$ and $b$.

The technique of matched asymptotic expansions has always seemed to me like black magic or, to use a different metaphor, tight-rope walking without a safety net. At some point I guessed that there might be a connection between (at least some instances of) this technique and geometric singular perturbation theory. I asked Walcher about this and he made an interesting suggestion. He proposed that what is being calculated in leading order is the point at which a solution of a slow-fast system hits the critical manifold (asymptotically) thus providing an initial condition for the slow system. I must try to investigate this idea more carefully.

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