MATH2640 Introduction to optimisation Semester One 201920
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MATH2640
Introduction to optimisation
Semester One 201920
1. (a) compute the directional derivative of f (x, g) = xg2 十 x3g at the point (4, -2) in
the direction u(^) = (1, 3).
(b) Find the equation of the tangent plane to the surface z = f (x, g) = x2 十 g2 , at the point (1, 2, 5), in the form ax 十 bg 十 cz = d.
(c) lf x,g and z are linked by the formula ①2 z 十 g = eg从 determine zx and zg via implicit diferentiation and use (one of) these to show that
2. cobb and Douglas (1928) deined their production function using data from the Amer- ican economy from 1899 to 1922, relating an output function Q = Q(k, L) to capital k and labour L, as follows
Q(k, L) = pkbLa
with itting coeicients p, a and b. using modern least-square data techniques, for another economy with data from 1899 to 1922 we found that p = 0.8, a = 0.8 and b = 0.25.
(a) Assuming a cost function of the form c(k, L) = 山kk 十 山LL and assuming a stable economy in equilibrium (i.e. the economy is assumed to be at the stationary point), and given the 1899 (equilibrium) data with Q* = 100, k* = 100, L* = 100 determine 山k , 山L in 1899 and calculate the cost (irst as formulas and second evaluated for the given values). ln 1922, for Q* = 240, k* = 480 and L* = 200, determine 山k , 山L again and calculate the cost.
(b) How well does the cobb-Douglas function it these data for Q (also give errors in percentages)? (Hints: 1001.05 必 126 and (480)0.25 (200)0.8 必 324.) ls the proit maximised at these two equilibria? calculate the Hessian and analyse it at the critical point. Discuss the validity of the cobb-Douglas function.
(c) lnterpret your results by comparing the relative changes of labour and capital costs, both as general formulas and for the particular values given, 山LL/c and 山kk/c , from 1899 to 1922. (Hint: show that the answers only depend on a and b.)
3. (a) consider a general quadratic form Q(x) = xTAx for an n-dimensional vector x. The principle minor test (PMT) for a symmetric matrix A concerns the following rules: (i) lf det(A) > 0 and all LPMs > 0 then Q is PD; (ii) if det(A) > 0, LPM1 < 0 and signs of LPMs alternate with order k, then Q is ND; etc. complete this PMT classiication of rules for all cases, including the cases with det(A) = 0 (so concerning the PSD, NSD and lD cases). Subsequently, for n = 2, consider Q(从, g) = a从2 + 2b从g + cg2 = a ((从 + bg/a)2 + (ca - b2 )(g/a)2 ), with a 0. Analyse the PMT for this quadratic form and prove that the PMT classiication (for n = 2, a < 0) is correct.
(b) consider the cobb-Douglas production function
Q(从,g, z) = 从1/4g1/4z1/4
subject to the budget constraint h(从,g, z) = a从 + bg + cz - d = 0, where a,b,c, d are positive constants. State the Lagrangian
L(从,g, z) = Q(从,g, z) - λh(从,g, z)
and deine λ . Find the maximum (从* , g* z* ) of Q in terms of these constants. Hence, ind expressions for the maximum value Q* and for the corresponding value λ* of the Lagrange multiplier (in terms of a,b,c, d). check also that the NDcQ is satisied.
(c) For the functions compared in (c), take a = b = c = 1 and d = 3 and use that for those values the stationary point 从* = g* = z* = 1 with Q* = 1. Derive the bordered Hessian of the Lagrangian deined above at this equilibrium by using that (here) Lxx = Qxx , Lxg = Qxg , etc. Demonstrate in detail whether the proit is maximum at this stationary point, or not.
4. consider the function f (x,g, 从) = x2g从2 subject to a single inequality constraint g(x,g, 从) =
3x+3g + 从 - 1 < 0, together with three NNc,s (non-negativity constraints) x,g, 从 > 0.
(a) what is the NDcQ? ls it satisied or not?
(b) state the KT-Lagrangian L(-)(x,g, 从, λ) and derive the corresponding KT-equalities and inequalities for this problem from the “standard” Lagrangian
L(x,g, 从, λ, μ1 , μ2 , μ3 ) = L(-)(x,g, 从, λ) + μ1x + μ2g + μ3 从
with multipliers λ, μ1 , μ2 , μ3 持 0 (Hint: eliminate μ1 , μ2 , μ3 ). That is, derive that the KT-equalities are:
xL-从 =0, gL-g = 0,
从Lz =0, λL) = 0,
and also provide the accompanying inequalities. calculate the KT equalities from the KT Lagrangian L(-)(x,g, 从, λ).
(c) Derive the stationary points, including (x* , g* , 从* ) = (2/15, 1/15, 2/5), and corre- sponding multiplier λ* and ind out which stationary point is a maximiser.
(d) using the standard Lagrangian of the problem, also derive (and complete) the general bordered Hessian HB for this problem:
Show that evaluation at the critical point (x* , g* , 从* ) yields:
Given that LPM4 = -64/28125, LPM3 = 72/125 argue whether the critical point is a minimum or maximum.
2023-08-22