MTH108: HOMEWORK 8 – CONTINUOUS POPULATION MODELS
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MTH108: HOMEWORK 8 – CONTINUOUS POPULATION MODELS
Please attempt sections A and B before the tutorials on Thursdays.
Section A – Warm-up questions
A1. Find the equilibria, check their stability and draw the phase portrait:
(i) = -x , (i i) = 4 - 2x , (i i i) = x2 - x , (i v) = x2 - 3x + 2 .
Section B – Main questions
B1. A small colony of yeast cells is grown in an experiment . The initial 20 cells multiply to 90 cells in 30 minutes . Assuming a continuous Malthusian model, calculate the intrinsic growth rate. Many hours later, the population has stabilized at around 500 cells . Propose a new continuous model for the population .
B2. Suppose that a population is modelled by the equation dx/dt = x3 - (K + 1)x2 + Kx , for K > 0. For the initial condition x(0) = x0 > 0, describe the behaviour of the solution x(t) as t - o.
B3. The population density of crabs in Yangcheng Lake obeys the equation
= 4x /1 - 、- 18 .
If x(0) = 20, then how will x(t) behave as t - o?
B4. Suppose the population density of eels in a lake is modelled by
= 24x /1 - 、- Ex ,
where E > 0 denotes the fishing effort . Calculate the fishing yield given that the population density has stabilized at 40 . What is the maximum sustainable yield?
B5. For each equation, find the values of r e R at which there is a change (a bifurcation) in the number or stability of the equilibria:
(i) = r - x2 , (i i) = rx - x2 , (i i i) = rx - x3 .
Section C – Extra problems
C1. For what values of r e R does the following equation have exactly 2 stable equilibria:
= r - x4 - 2x3 + 35x2 .
C2. Use the substitution u(t) = 1/x(t) to solve the Verhulst equation dx/dt = rx(1 - x/K) .
C3. Consider the model
= rx2 /1 - 、- Ex ,
with r, K, E > 0 . What is the maximum value of E for which fishing is sustainable? Does it give the best yield?
2023-04-22