Math 142 Test 1 2021
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Test 1
1. Suppose a system of fish and sharks satisfies
= F(120 − 12S)
= S(−6 + 3F)
(a) (5 points) Find the interior critical point (F*, S*) and linearize by substituting
F(t) = F* + εX(t) S(t) = S* + εY(t)
(b) (7 points) solve the linearization to approximate F(t) and S(t) when ε is small · Assume X(0) = 1,Y(0) = 0 ·
(c) (4 points) what is the approximate period of oscillation when ε is small?
Test 2
2 · (8 points) suppose:
· ① is a population of worms that grows exponentially in the absence of other species.
· g is a population of beetles that grows logistically·
· 2 is a plant that grows logistically. It has a mutually benefcial relationship with beetles which pollinate the plant , but is not efected by the other animals.
· U is a bird that eats many things in the environment and grows logistically, but also eats worms
· w is a weasel that survives on eating birds and beetles.
write a Lotka-volterra system that could describe this.
3. Consider the competing species equations
= x(8 − 2x − y )
dt(dy) = y(6 − x − y − z)
dt(dz) = z(9 − 4y − z)
(a) (4 points) what is the relationship between x, y, z in this system? Give an example
of 3 species that would satisfy this relationship, and indicate why you chose those species.
(b) (5 points) Find the interior critical point and the Jacobian J at this critical point by using that this is a kolmogorov system. (Do not take any derivatives. )
(c) (5 points) The characteristic polynomial of J is det(λI − J) = λ3 + 9λ2 + 6λ − 42 · Determine if it is possible for all species to coexist.
(d) (6 points) set 2 = 0 and sketch the general olution to the system in the ①g-plane.
e) (4 points) suppose ①(0)’g(0)’2(0) 米 0. Determine ifit's possible for 2 to go extinct , while both ① and g coexist.
(f) (6 points) set ① = 0 and sketch the general solution to the system in the g2-plane.
(g) (4 points) sketch g(t) and 2(t) if ①(0) = 0’g(0) = 3 2(0) = 1
(h) (4 points) suppose ①(0)’g(0)’2(0) 米 0. Determine if it's possible for ① and 2 to go extinct , while g survives.
Test 3
4. suppose a population u(x, t) of fsh living
the reaction-difusion eauation
= −
(a) (4 points) Find a phase-plane system
u(x, t) = U(x − ct) ·
in an infnite river (−∞ < x < ∞) satisfes
u3 + 3u2 − 2u.
for traveling wave solutions
(b) (5 points) sketch the nullclines and direction arrows of the phase-plane system.
(c) (5 points) Find all critical points and the eigenvalues ofthe Jacobian at each critical point.
(d) (4 points) Find a value cmin such that the system has no spiral sinks for c ≥ cmin ·
(e) (8 points) sketch the phase~plane solution in the UV-plane for c < cmin ·
(f) (8 points) sketch all possible traveling wave solutions for c > cmin. Explain the meaning of each in terms of what happens to the population on the interval −∞ < x < ∞ over time.
(g) (4 points) Are there any physically relevant traveling waves for c < cmin ?Explain your reasoning for why or why not .
2023-07-26