Systems Biology (Methods in Molecular Biology)

d

ds

¼τ

d

dt

:

This procedure consists in rescaling the time variable in order to
zoom in on the system process during the first temporal period.
Passing fromttos, the system of ODEs becomes

du

ds

þu¼w,

dw

ds

þτw¼τλguðÞ,

and its limit asτ! 0 +is formally given by

du

ds

þu¼w,

dw

ds

 0 : ð 11 Þ

The second equation in Eq.11 does trivially express the fact that
wis, at first glance, independent ofs(i.e., constant). Therefore, the
corresponding approximated solutions to Eq.11 are

du

ds

þuw 0 , ww 0 ,

and, by applying classical results on explicit solutions to linear
ODEs [51], one ultimately obtains

uðsÞw 0 þðu 0 w 0 Þes, wðsÞw 0 ,

for some initial conditionsu(0)¼u 0 andw(0)¼w 0. Finally, in a
fast time-scale (s!þ1or, equivalently,τ! 0 +) the solution gets
closer and closer to the straight linew¼u. The “fate” of the system
is not yet decided, but it appears to be dictated only by the variable
u, whose dynamics is determined at a slower time-scale. Coming
back to Eq.7 and putting formallyτ¼0, we deduce that the slow
dynamics is described by the reduced system

u¼w,

dw

dt

þw¼λguðÞ, ð 12 Þ

which corresponds to the scalar equation

du

dt

þuλguðÞ¼ 0 : ð 13 Þ

The characterization of the equilibria for Eqs.12–13 and their
stability analysis is precisely what has been performed above, recal-
ling thath(u;λ)¼uλg(u).
A remarkable fact is that system Eq.7 possesses an alternative
representation consisting of a single second-order differential equa-
tion, which can be obtained by differentiating the first equation
of Eq.7 with respect to timetand taking advantage of the second
equation, so that

Mathematical Modeling of Phase Transitions in Biology 117