MATH0030 Problem Sheet 8
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MATH0030 Problem Sheet 8
1. Consider the planar system
x˙ = x(a 一 bx 一 cy) (1) y˙ = y(d 一 ex 一 fy) (2)
where a, b, c, d, e > 0 and fb > ce. Given that there exists a unique interior steady state (x* , y* ) find a Lyapunov function V of the form
V (x, y) = α(x 一 x* )2 + β(x 一 x* )(y 一 y* ) + γ(y 一 y* )2 .
2. Consider the chemostat model
S˙ = α(S〇 一 S) 一
x˙ = x ╱ 一 α、
where S〇 , α, a, m > 0 are constants.
(a) Can the function Φ = S + x 一 S〇 be a Lyapunov function?
(b) Rewrite the system using variables Φ and x.
(c) Sketch the phase protrait for the new coordinates (Φ, x)
3. Consider the generalized Lotka - Volterra system
x˙ = x(1 一 x 一 αy 一 βz)
y˙ = y(1 一 βx 一 y 一 αz)
z˙ = z(1 一 αx 一 βy 一 z)
where α, β > 0 and α + β = 2.
(a) Write the interaction matrix. What kind of interactions are taking place in this system?
(b) Let
V (x, y, z) = x + y + z,
Obtain a differential equation for V and show that V (x(t), y(t)) → 1 as t → &.
(c) Show also that S˙ = 3S(1 一 V) with S(x, y, z) = xyz . What is the asymptotic behaviour of S?
(d) Sketch the surfaces S(x, y, z) = const and V (x, y, z) = 1 and show that they may intersect in a closed curve. What can you say about the solution (x(t), y(t), z(t)) as t → &?
2023-05-15