 Mathematics and Computational Methods for Complex Systems summative-assessment.pdf Prof. Luc Berthouze
Introduction
This coursework is worth 100% and is due Thursday, 6 January 2022, 16:00 (but
please always check what Canvas and Sussex Direct says!). Submission is electronic, via Canvas. Submission guidelines are identical to the formative coursework,
namely: (a) Analytical work can be hand-written or typeset; (b) simulation code +
analysis can be presented separately or integrated into a Jupyter / Colab notebook
(whatever you prefer!). Unlike the previous coursework, there is no page limit but
conciseness of exposition is always best!
In this coursework, you are going to engage in some proper modelling work. You are
given a scenario along with a couple of ordinary differential equations that describe
the operation of this system (in principle I could have asked you to derive them from
the scenario description but I don’t). The questions take you through a number of
steps by which you will gain a detailed understanding of the system / model. Although
all questions are related, it is possible to get results even if you find yourself unable
to complete some of the steps.
Statement of problem
We consider a system of individuals (the nature of which is irrelevant) that can take
two states A and B. At any point in time, an individual can only be in one state.
There are no other states possible. We will denote by [A] the number of individuals
in state A and by [B] the number of individuals in state B. There are N individuals
in total. All individuals can be assumed to be in contact with all other individuals. At
time t = 0, there are B0 individuals in state B. All other individuals are therefore in
state A. Individuals in state A turn into state B at a rate β [B]
N , i.e., proportional to the
number of their neighbours that are in state B. Individuals in state B turn into state A
at a fixed rate γ.
Now, in an ideal world, you should be able to write the two ordinary differential equations (ODE) that describe this system. To make sure that everyone can proceed with
the coursework, I am providing the ODEs in the last page of this document. Before
consulting that section, you are strongly encouraged to try and come up with your
own formulation.
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Mathematics and Computational Methods for Complex Systems summative-assessment.pdf Prof. Luc Berthouze
Questions
Analytical work
1. Using the fact that [A] + [B] = N at all times, write ˙ [B] as a function of [B], i.e.,
the expression should no longer involve [A]. This is your (so-called) mean-field
equation.
(2 marks)
2. Find the equilibria of the system and determine their stability. From now on,
we will refer to the non-zero equilibrium as B∗. You may find it useful to write
your results in terms of the following quantity R0 = β
γ . Plot the phase portrait,
i.e., [B] vs [A], identifying the equilibria and their stability (following convention
described in the 2nd synchronous lecture of Unit 5).
(8 marks)
3. Produce the bifurcation plot for this system, that is, plot the value of the equilibria
as a function of R0, with R0 taking values from 0.1 to 5.0. For this question, the
value of N is irrelevant (provided it’s strictly positive) so use 1000 for example.
This should be done using Python.
(4 marks)
4. In this question, you are going to integrate ˙ [B] analytically to obtain an expression for [B](t), i.e., an expression that gives the number of individuals in state
B in time. It is rarely the case that this can be done but with this system, it is
possible. You will do this in four steps:
• Starting from the mean-field equation, factorise the right hand side by [B]
2,
then write an expression for 1
[B]2 ˙ [B]. (4 marks)
• Consider the following variable substitution: y = 1
[B]
. Using the chain rule,
express y˙ in terms of [B], then derive a simple expression for y˙, i.e., this expression should only involve y terms and parameters of the system. There
shouldn’t be any [B] or [A]. However, it will be helpful to use B∗ (calculated
in the 2nd question) to simplify the expression. (4 marks)
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Mathematics and Computational Methods for Complex Systems summative-assessment.pdf Prof. Luc Berthouze
• Integrate this equation. You should be able to do this without any help, but
if help is needed, you should note that this expression looks very much like
the equation we solved during a synchronous lecture in Unit 4, replacing λ
and I by appropriate quantities. You can then use the result to derive an
equation for y(t). Please see short document summarising the derivation
from the lecture. (4 marks)
• You can now produce a fully worked out expression for [B](t) by remembering that [B] = B0 at time t = 0. (4 marks)
NB: In going through this question, do make sure to consider all scenarios possible regarding the value of R0.
5. Using different values of B0 (between 1 and N – briefly discuss the case B0 = 0),
plot solutions of [B](t) for various values of R0 between 0.1 and 5.0 (with γ = 0.5
for example). Confirm your expression for [B](t) is correct by (a) verifying that
it converges to B∗ for large times t and (b) visually confirming agreement when
integrating the mean-field equation using Euler (use Python). What happens
when R0 = 1? Speculate as to what this means. We will get back to this. For a
given value of R0, what happens when the value of γ changes? Provide a brief
explanation.
(11 marks)
Simulation work
Some more could be done analytically but let’s now turn to simulations. For this
part of the work, we will employ the Gillespie algorithm, an algorithm often used to
simulate complex systems. There is something very important to understand here.
The mean-field equation provided to you is derived from considering the interactions
of a very large number of individuals. The Gillespie algorithm, instead, provides
discrete simulations of the system with few individuals by explicitly simulating every
transitions. In other words, a single run of the Gillespie algorithm represents one
sample trajectory of all possible trajectories for this system. In principle, the average
of a (sufficiently) large number of Gillespie runs should converge to the mean-field.
This is what you will test here. NB: Gillespie provides a mathematically rigorous and
computationally efficient alternative to agent-based modelling. You are not asked to
compare the two approaches even if that would be an interesting thing to do (NB: but
certainly not within this assessment!).
To make sure everybody can work, I provide a Python implementation of the Gillespie
algorithm for this problem. Use the code although feel free to write your own if you feel
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Mathematics and Computational Methods for Complex Systems summative-assessment.pdf Prof. Luc Berthouze
so inclined – you would need to read up on the Gillespie algorithm obviously. Function
gillespie_ABA takes 5 arguments: N, B0, β, γ and Tmax (max simulation time) and
produces three outputs: T, A and B which contain the time of each transition (when
one, and only one, individual changed state, whatever that state was), the numbers
of individuals in state A and in state B after that transition, respectively. NB: The
Gillespie algorithm uses exponentially-distributed times to next event so each run will
have a different timeline T of events. This won’t be apparent to you if plotting with
lines but will matter in what comes below.
1. Explore the behaviour of the system when considering suitably chosen scenarios, i.e., focus on the limit cases (e.g., small R0, large R0 and R0 = 1; small
N, large N; small B0, large B0). For each scenario, use the code provided to
generate many realisations of the stochastic process. Plot all realisations on a
single plot. Make relevant qualitative observations.
(10 marks)
2. For each scenario, calculate the average (and standard deviation) of the realisations. Here, you are going to face a problem linked with the nota bene from
the introductory paragraph. You will need to think of a solution and implement
it. Superimpose the average (and error bars) to the realisations. Use a larger
line width for visibility.
(11 marks)
3. Finally, superimpose the mean-field solution. Again, use a larger line width and
different colour for visibility. Describe and interpret agreement between average
of stochastic realisations and mean-field in relation to the choice of parameters.
In this question, using B0 = 1 (i.e., only one individual in state B at t = 0) can
You may want to refer to your bifurcation plot.
(8 marks)
4. [Slightly challenging question]: Consider 100 replications for N = 1000, β =
0.51, γ = 0.5 and 100 replications for N = 1000, β = 0.95, γ = 0.5. You should
notice a substantial difference in agreement between the mean-field and the average of the stochastic realisations depending on which scenario is considered.
How could you improve agreement for the scenario with the poorest agreement.
Please note: The difference in B∗ is not the quantity of interest here. Rather
you should think about why agreement is so poor. This does not actually involve
solution and provide evidence of improved agreement.
(12 marks)
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Mathematics and Computational Methods for Complex Systems summative-assessment.pdf Prof. Luc Berthouze
A bit of critical thinking
In this last part of the coursework, you are invited to think a bit more about what you
have just done.
1. So far the brief has provided no information whatsoever regarding what the
states A and B are and what the individuals are. Thinking about what is happening in this system, provide at least one example of real-world scenario to
which this model could apply. Bonus points will be given for any answer that
provides two examples, one in which the equilibria are of interest and the other
in which the critical regime (when R0 = 1) is of interest. Either way, what is the
benefit of being able to model the system?
(8 marks)
2. [Very tough question]: The model provided implicitly assumes that all individuals are potentially in contact with each other. What would be a more likely
scenario? What changes would have to be made to the code of the Gillespie
algorithm in order to include such a scenario? If you are able to do this, do
it. Then, speculate as to what could affect the results observed in the previous
questions. If you feel so inclined, demonstrate it experimentally. NB: Only 10
marks have been given to this question. However, anyone managing it successfully would receive an extra 10 marks for the assignment (with the total
mark capped to 100 obviously).
(10 marks)
Total marks: 100.
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Mathematics and Computational Methods for Complex Systems summative-assessment.pdf Prof. Luc Berthouze
The ODEs
The system is fully described by the following two ordinary differential equations:
˙ [B] = β [B]
N [A] − γ[B]
˙ [A] = −β [B]
N [A] + γ[B]
where β and γ are strictly positive real numbers. The first equation provides the rate
of change of the number of individuals in state B and says that it is the sum of the
number of individuals in state A that become B with a rate β [B]
N (proportional to the
number of other individuals in state B) minus the number of individuals in state B that
turn to state A with rate γ. I leave it to you to interpret the second equation. Summing
both equations lead to 0 which is consistent with the fact that the total population
[A] + [B] = N at all times. To complete the description we need the initial conditions.
We will call B0 the number of individuals in state B at t = 0. The number of individuals
in state A is therefore N − B0.
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