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M. Golubitsky, I. Stewart, J. Best, H.F. Nijhout and M. Reed

Homeostasis with multiple inputs

*Preprint.*
To appear.

Homeostasis is a regulatory mechanism whereby some output variables
of a system are kept approximately constant as input parameters vary over some region.
Important applications include biological and chemical systems. Golubitsky and Stewart
reformulated homeostasis in the context of singularity theory by replacing
`approximately constant over an interval' by `zero derivative with respect to the
input at a point', and discussed coordinate changes that put the singularity in
normal form. We call this form of homeostasis infinitesimal homeostasis.
(Reed et al. show by example that biologically relevant
biochemical mechanisms can exhibit homeostasis even though infinitesimal homeostasis
does not occur for realistic model parameters.)
The main focus in Golubitsky and Stewart was on systems with one input and one output variable,
classified by a subfamily of the elementary
catastrophes of and Zeeman.
In this paper we use the singularity theory approach to study simultaneous homeostasis
for two input parameters.
As before, coordinate changes that preserve homeostasis play a prominent role
because such coordinate changes provide the basis for singularity theory.
We classify the singularities concerned and discuss their effect on homeostasis. We also speculate
on why homeostasis in models may look as though it comes from infinitesimal homeostasis, even when technically it does not. This speculation uses the classification of elementary catastrophes.