Advanced Mathematical Economics 1
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Advanced Mathematical Economics
Part A
Suppose that half of the houses in a city are in polluted areas. Residents in polluted areas suffer health problems, and can only do unskilled work. Apart from this problem, all workers can do skilled and unskilled work. A firm hires skilled and unskilled work-
ers to make furniture. Workers are endowed with a house and hours which they can sell. Workers can buy houses and furniture. Workers can not live together (due to fire regulations).
(i) Formulate a competitive model of the housing, labour and furniture markets.
(ii) Reformulate the worker’s problem using a Bellman equation with a a housing choice, and with the other choices buried inside a value function.
(iii) Write down a formula for the marginal profit of a skilled wage increase.
(iv) Suppose that the firm’s production function is strictly concave. Prove that the firm has at most one optimal choice.
Part B
(i) (easy) Let f : X → Y be a continuous function between two metric spaces (X, dX ) and (Y, dY). Prove or disprove that f (∂A) = ∂f (A) for all sets A ⊆ X . Note: ∂A denotes the set of boundary points of A, and f (A) = {f (a) : a ∈ A} .
(ii) (easy) Consider the metric space (R, d), where d is the discrete metric. Find a contraction f : R → R on this space.
(iii) (easy) Consider the sequence fn : N → [0, 1] defined by
Prove that this sequence is not convergent in (B(N), d∞).
(iv) (medium) Find a metric d such that the sequence fn in the previous question converges to f∗ (x) = 1.
(v) (medium) Let X = [0, 1). Find a metric d such that (X, d) is a compact metric space.
(vi) (medium) Suppose you are considering buying a house at market price p, which you value at v. But you don’t want to buy if it has any (major) defects. You have taken a quick look already, and you think the probability of defects is q. You can pay inspectors c for conditionally independent reports about the house, which have type 1 and 2 errors of x and y. Each day, you choose whether to buy the house,
to buy another report, or to give up. You discount days at rate β. You have a Bellman equation
V(q) = max {0, qv − p,
Prove that the optimal policy involves giving up for low q, i.e. there exists q1 ∈ [0, 1] such that giving up is optimal for all q ∈ [0, q1].
(vii) (hard) Define
Prove or disprove: if f : [0, 1] → [0, 1], f (1) = 1, and f (x) > x for all x < 1, then limn→∞ fn(0) = 1.
(viii) (hard) Suppose (X, dX ) and (Y, dY) are metric spaces, where K = X ∩ Y is a compact set in both spaces. Suppose that dX (a, b) = dY (a, b) for all a, b ∈ K. Let Z = X ∪ Y. Construct a metric dZ on Z such that dZ (a, b) = dX (a, b) for all a, b ∈ X, and dZ (a, b) = dY (a, b) for all a, b ∈ Y . Hint: use the fact that dX and dY are continuous. Note: a complete proof is long, with lots of cases to consider. You can get an almost perfect score for Part B by showing 1 or 2 cases well.
2021-12-23