# Multi-variable Calculus and Optimization

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1 Multi-variable Calculus and Optimization Dudley Cooke Trinity College Dublin Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 1 / 51

2 EC2040 Topic 3 - Multi-variable Calculus Reading 1 Chapter , 8 and 11 (section 6 has lots of economic examples) of CW 2 Chapters 14, 15 and 16 of PR Plan 1 Partial differentiation and the Jacobian matrix 2 The Hessian matrix and concavity and convexity of a function 3 Optimization (profit maximization) 4 Implicit functions (indifference curves and comparative statics) Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 2 / 51

3 Multi-variable Calculus Functions where the input consists of many variables are common in economics. For example, a consumer s utility is a function of all the goods she consumes. So if there are n goods, then her well-being or utility is a function of the quantities (c 1,..., c n ) she consumes of the n goods. We represent this by writing U = u(c 1,..., c n ) Another example is when a firm s production is a function of the quantities of all the inputs it uses. So, if (x 1,..., x n ) are the quantities of the inputs used by the firm and y is the level of output produced, then we have y = f (x 1,..., x n ). We want to extend the calculus tools studied earlier to such functions. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 3 / 51

4 Partial Derivatives Consider a function y = f (x 1,..., x n ). The partial derivative of f with respect to x i is the derivative of f with respect to x i treating all other variables as constants and is denoted, f x i or f xi Consider the following function: y = f (u, v) = (u + 4) (6u + v). We can apply the usual rules of differentiation, now with two possibilities: f = 1 (6u + v) + 6 (u + 4) = 12u + v + 24 u f = 0 (6u + v) + 1 (u + 4) = u + 4 v Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 4 / 51

5 Partial Derivatives - Economic Examples Again, suppose y = f (x 1,..., x n ). Assume n = 2 and x 1 = K and x 2 = L; that is, y = f (K, L). Then assume the following functional form: f (K, L) = K 0.25 L 0.75 ; i.e., a Cobb-Douglas production function. The partial derivatives are given by, f K = 0.25K 0.75 L 0.75, f L = 0.75K 0.25 L 0.25 Alternatively; suppose utility is U = ca α cb 1 α, where a =apples, b =bananas. We find: ( ) U 1 α cb = α, U ( ) α ca = (1 α) c a c b c a So, for a given labor input, more capital raises output. For given consumption of apples, consuming more bananas makes you happier. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 5 / 51 c b

6 Interpretation of Partial Derivatives Mathematically, the partial derivative of f with respect to x i tells us the rate of change when only the variable x i is allowed to change. Economically, the partial derivatives give us useful information. general terms: 1 With a production function, the partial derivative with respect to the input, x i, tells us the marginal productivity of that factor, or the rate at which additional output can be produced by increasing x i, holding other factors constant. 2 With a utility function, the partial derivative with respect to good c i tells us the rate at which the consumer s well being increases when she consumes additional amounts of c i holding constant her consumption of other goods. I.e., the marginal utility of that good. In Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 6 / 51

7 Derivative of a Linear Function As with functions of one variable, to gain some intuition, we start with functions that are linear. We motivated the notion of a derivative by saying that it was the slope of the line which looked like the function around the point x 0. When we have n variables, the natural notion of a line is given by y = a 0 + a 1 x a n x n When there are two variables, x 1 and x 2, the function y = a 0 + a 1 x 1 + a 2 x 2 is the equation of a plane. In general, the function y = a 0 + a 1 x a n x n is referred to simply as the equation of a plane. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 7 / 51

8 Two Variable Linear Function Consider the plane y = a 0 + a 1 x 1 + a 2 x 2. How does the function behave when we change x 1 and x 2? Clearly, if dx 1 and dx 2 are the amounts by which we change x 1 and x 2, we have, dy = a 1 dx 1 + a 2 dx 2 Note furthermore that the partials are, We can then write, y = a 1 and y = a 2 x 1 x 2 dy = y x 1 dx 1 + y x 2 dx 2 Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 8 / 51

9 Jacobian Matrix We can take this result (i.e., that dy = y x 1 dx 1 + y x 2 dx 2 ) and recast it in matrix (here, vector) form. [ ] [ ] dy = y y dx1 x 1 x 2 } {{ } dx 2 J We call J the Jacobian. [ y x 1 Take our previous example. The vector what we can think of as the derivative of the plane. y x 2 ] = [ a 1 a 2 ] is This vector tells us the rates of change in the directions x 1 and x 2. The total change is therefore computed as a 1 dx 1 + a 2 dx 2. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 9 / 51

10 General Functions Now consider a more general two variable function, y = f (x 1, x 2 ). With a general function y = f (x 1, x 2 ), the idea is to find a plane which looks locally like the function around the point (x 1, x 2 ). Since the partial derivatives give the rates of change in x 1 and x 2, it makes sense to pick the appropriate plane which passes through the point (x 1, x 2 ) and has slopes f / x 1 and f / x 2 in the two directions. The derivative (or the total derivative) of ] the function f (x 1, x 2 ) at y (x 1, x 2 ) is simply the vector x 2 where the partial derivatives [ y x 1 are evaluated at the point (x 1, x 2 ). We can interpret the derivative as the slopes in the two directions of the plane which looks like the function around the point (x 1, x 2 ). Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 10 / 51

11 An Example When we have a function y = f (x [ 1,..., x n ), the derivative at (x 1,..., x n ) is simply the vector y y x 1... x n ]. Take a more concrete example. Suppose, y = 8 + 4x x 2 x 3 + x 3 The Jacobian must be, [ 8x1 6x 3 1 ] = [ y x 1 y x 2 ] y x 2 So we can write, dy = [ 8x ] dx 1 dx 2 dx 3 Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 11 / 51

12 An Economic Example Consider the utility function from before; U = ca α cb 1 α, where a =apples, b =bananas. The Jacobian is a vector of the partials (which we already computed). The change in utility is therefore given by the following. [ ] [ ] du = U U dc a c a c b dc b [ ( ) 1 α ( ) α ] [ ] = α cb c a (1 α) ca dc a c b dc b The same is true for the Cobb-Douglas production function, f (K, L) = K 0.25 L Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 12 / 51

13 Second-Order Partial Derivatives We now move on to second-order partial derivatives. will be economic problems/interpretation. Our motivation Given a function f (x 1,..., x n ), the second-order derivative 2 f x i x j is the partial derivative of f x i with respect to x j. The above may suggest that the order in which the derivatives are taken matters: in other words, the partial derivative of f x i with respect to x j is different from the partial derivative of f x j with respect to x i. While this can happen, it turns out that if the function f (x 1,..., x n ) is well-behaved then the order of differentiation does not matter. This result is called Young s Theorem. We shall only be dealing with well-behaved functions (i.e., Young s Theorem will always hold). Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 13 / 51

14 Economic Example For the production function y = K 0.25 L 0.75, we can evaluate the second-order partial derivative,, in two different ways. 2 y K L First, since y K = 0.25K 0.75 L 0.75, taking the partial derivative of this with respect to L, we get, y L K = 3 16 K 0.75 L 0.25 And since f L = 0.75K 0.25 L 0.25, we have, y K L = 3 16 K 0.75 L 0.25 This illustrates Young s Theorem: no matter in which order we differentiate, we get the same answer. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 14 / 51

15 Economic Interpretation The second-order partial derivatives can be interpreted economically. Our example production function is y = K 0.25 L Therefore, 2 y K 2 = 3 16 K 1.75 L 0.75 This is negative, so long as K > 0 and L > 0. This tells us that the marginal productivity of capital decreases as we add more capital. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 15 / 51

16 Economic Interpretation As another example, consider, 2 y K L = 3 16 K 0.75 L 0.25 This is positive, so longas K > 0, L > 0. This says that the marginal productivity of labour increases as you add more capital. It also suggests a symmetry. The above can be interpreted as saying that the marginal productivity of capital increases when you add more labor. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 16 / 51

17 Concave and Convex Functions Our interest in these SOCs is motivated by the local versus global max/min of functions. Previously, we related this to concavity and convexity. In the case of one variable, we defined a function f (x) as concave if f (x) 0 and convex if f (x) 0. Notice that in the single-variable case, the second-order total is, d 2 y = f (x)(dx) 2 Hence, we can (equivalently) define a function of one variable to be concave if d 2 y 0 and convex if d 2 y 0. The advantage of writing it in this way is that we can extend this definition to functions of many variables. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 17 / 51

18 Second-Order Total Differential Suppose y = f (x 1, x 2 ). The first order differential is dy = f x1 dx 1 + f x2 dx 2. Think of dy as a function. It s differential is, d(dy) = [f x1 x 1 dx 1 + f x1 x 2 dx 2 ] dx 1 + [f x1 x 2 dx 1 + f x2 x 2 dx 2 ] dx 2 Collecting terms, d 2 y = f x1 x 1 (dx 1 ) 2 + 2f x1 x 2 dx 1 dx 2 + f x2 x 2 (dx 2 ) 2 The second-order total differential depends on the second-order partial derivatives of f (x 1, x 2 ). Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 18 / 51

19 Second-Order Total Differential Corollary: If we have a general function y = f (x 1,..., x n ), one can use a similar procedure to get the formula for the second-order total differential. This is a little more complicated but it can be written compactly as, d 2 y = n n i=1 j=1 f xi x j dx i dx j E.g., y = f (x 1, x 2, x 3 ) the expression becomes, d 2 y = f x1 x 1 (dx 1 ) 2 + f x2 x 2 (dx 2 ) 2 + f x3 x 3 (dx 3 ) 2 + 2f x1 x 2 dx 1 dx 2 + f x1 x 3 dx 1 dx 3 + f x2 x 3 dx 2 dx 3 Again, we care about this because a function y = f (x 1,..., x n ) will be concave if d 2 y 0 and convex if d 2 y 0. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 19 / 51

20 Maxima/Minima/Saddle Points As in the single variable case, we are really after maxima and minima. The first order conditions alone cannot distinguish between local maxima and local minima. Likewise, the first order conditions cannot identify whether a candidate solution is a local or global maxima. We thus need second order conditions to help us. For a point to be a local maxima, we must have d 2 y < 0 for any vector of small changes (dx 1,..., dx n ); that is, y needs to be a strictly concave function. Similarly, for a point to be a local minima, we must have d 2 y > 0 for any vector of small changes (dx 1,..., dx n ); that is, y needs to be a strictly convex function. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 20 / 51

21 Hessian Matrix - Two Variable Case As things stand, none of this is not particularly useful since it is not clear how to go about verifying that the second-order total differential of a function of n variables is never positive or never negative. However, notice that we can write the second order differential of a function of two variables (that is, d 2 y = f 11 (dx 1 ) 2 + 2f 12 dx 1 dx 2 + f 22 (dx 2 ) 2 ) in matrix form in the following way. d 2 y = [ dx 1 dx 2 ] [ f 11 f 12 f 12 f 22 ] [ dx1 dx 2 [ ] f11 f The matrix 12 is called the Hessian matrix, H. f 12 f 22 the matrix of second-order partial derivatives. ] It is simply Whether d 2 y > 0 or d 2 y < 0 leads to specific restrictions on the Hessian. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 21 / 51

22 Hessian Matrix - General Case In the general case, the second order differential can be written as, f 11 f f 1n dx 1 d 2 y = [ ] dx 1 dx 2... dx n f 12 f f 2n dx f 1n f 2n... f nn dx n If we want to know something about d 2 y all we need to do is evaluate the Hessian. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 22 / 51

23 Hessian Matrix - Economic Example Returning to the Cobb-Douglas utility case; U = ca α c β b, where a =apples, b =bananas. The first-order differential was, [ ] [ ] du = αca α 1 c β b βca α c β 1 dc a b dc b The second-order differential is, c a c a 2 U c b c a d 2 U = [ dc a dc b ] [ 2 U 2 U c a c b 2 U c b c b ] [ dca dc b ] where, [ 2 U c a c a 2 U c b c a 2 U c a c b 2 U c b c b ] = [ α (α 1) ca α 2 c β b αβca α 1 αβc α 1 a c β 1 b c β 1 b β (β 1) c α a c β 2 b ] Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 23 / 51

24 Positive Definite and Negative Definite Matrices The Hessian matrix is square. We can say something about square matrices: A symmetric, square matrix, A n, is said to be positive definite if for every column vector x = 0 (dimension 1 n) we have x T Ax > 0. A symmetric, square matrix A n is negative definite if for every column vector x = 0 (dimension 1 n), we have x T Ax < 0. To check for concavity/convexity we evaluate the Hessian. Hessian (for our purposes) can be PD or ND. But the If the Hessian is ND the function is concave. If the Hessian is PD the function is convex. All we need to be able to do then is detemine whether a given matrix is ND or PD. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 24 / 51

25 Checking for PD-ness and ND-ness a 11 a a 1n Let A = a 12 a a 2n be a n n symmetric square matrix. a 1n a 2n... a nn The principal minors of A are the n determinants, a 11, a 11 a 12 a 12 a 22, a 11 a 12 a 13 a 12 a 22 a 23 a 13 a 23 a 33,..., A The matrix A is positive-definite if and only if all the principal minors are positive. The matrix A is negative-definite if and only if its principal minors alternate in sign, starting with a negative. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 25 / 51

26 Examples of PD and ND Matrices One can see how this works if we look at the matrices I n and I n. The principal minors of I are uniformly 1; on the other hand the principal minors of I start with 1 and then alternate in sign. Thus, the matrices I n and I n are trivially positive-definite and negative-definite. [ ] 1 1 Consider,. One can easily show that the matrix is 1 4 positive-definite. How? Well, the principle minors are 1 and 3. [ ] 1 1 The matrix is negative-definite as the principle minors 1 4 are 1 and 3. A matrix may be neither positive-definite [ ] nor negative-definite. 1 1 instance, consider the matrix. 1 4 For Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 26 / 51

27 ND and PD Relation to Concavity and Convexity We defined a function to be concave if d 2 y 0 for all x and to be convex if d 2 y 0 for all x. If the function satisfies a stronger condition d 2 y < 0 for all x then it will be said to be strictly concave. Analogously, if d 2 y > 0 for all x, then it will be said to be strictly convex. We can also relate the notions of positive-definiteness and negative-definiteness to strict concavity and strict convexity. 1 A function f is strictly concave if the Hessian matrix is always negative definite. 2 A function f is strictly convex if the Hessian matrix is always positive definite. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 27 / 51

28 Example Consider the following. Is f (x, y) = x 1/4 y 1/2 concave or convex over the domain (x, y) 0? Compute the second order partials. E.g., f xy (x, y) = f yx (x, y) = (1/8) x 3/4 y 1/2 and f xx (x, [ y) = (3/4) (1/4) x 7/4 y 1/2. So, (3/4) (1/4) x H = 7/4 y 1/2 (1/8) x 3/4 y 1/2 ] (1/8) x 3/4 y 1/2 (1/4) x 1/4 y 3/2 The principle minors are, (3/4) (1/4) x 7/4 y 1/2 < 0 and, (3/4) (1/4) x 7/4 y 1/2 (1/8) x 3/4 y 1/2 (1/8) x 3/4 y 1/2 (1/4) x 1/4 y 3/2 = (3/64) x 7/4 y 1/2 x 1/4 y 3/2 (1/64) x 3/4 y 1/2 x 3/4 y 1/2 [ = (1/64) 2x 6/4] /y > 0 We conclude x 1/4 y 1/2 is convex over the domain (x, y) 0. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 28 / 51

29 Unconstrained Optimization with Multiple Variables We now link concavity and convexity to optimization problems in economics. Consider the general maximization problem, max f (x 1,..., x n ) The first order conditions for maximization can be identified from the condition that the first order differential should be zero at the optimal point. That is, a vector of small changes (dx 1,..., dx n ) should not change the value of the function. We thus have dy = f x1 dx 1 + f x2 dx f xn dx n = 0 Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 29 / 51

30 First Order Conditions Suppose that we have a function of 2 variables y = f (x 1, x 2 ). Then, the first order condition dy = 0 implies that f x1 dx 1 + f x2 dx 2 = 0. Therefore, the only way we can have f x1 dx 1 + f x2 dx 2 = 0 is when f x1 = 0 and f x2 = 0. By the same logic, for a general function y = f (x 1,..., x n ), the first order conditions are f x1 = 0, f x2 = 0,..., f xn = 0 Note that these conditions are necessary conditions in that they hold for minimization problems also. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 30 / 51

31 Relation to SOCs for Optimization We can now summarize the first and second order conditions for an optimization problem in the following way: If (x 1,..., x n ) is a local maximum, then the first order conditions imply that f i (x 1,..., x n ) = 0 for i = 1,..., n. The second order condition implies that the Hessian matrix evaluated at (x 1,..., x n ) must be negative definite. If (x 1,..., x n ) is a local minimum, then the first order conditions imply that f i (x 1,..., x n ) = 0 for i = 1,..., n. The second order condition implies that the Hessian matrix evaluated at (x 1,..., x n ) must be positive definite. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 31 / 51

32 Economic Example: Profit Maximization To better understand all of this, we can look at a firm s optimization problem. Suppose a firm can sell it s output at \$10 per unit and that it s production function is given by y = K 1/4 L 1/2. What combination of capital and labour should the firm use so as to maximize profits assuming that capital costs \$2 per unit and labour \$1 per unit? The firm s profits are given by Π(K, L) = 10y 2K L = 10K 1/4 L 1/2 2K L } {{ } revenue minus costs This is a problem of unconstrained optimization with multiple variables. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 32 / 51

33 Economic Example Continued We can use the first order conditions to obtain potential candidates for optimization, as with one variable. For the firm we have the following profit maximization problem: The first order conditions are Π(K, L) = 10K 1/4 L 1/2 2K L Π K = (5/2)K 3/4 L 1/2 2 = 0 Π L = 5K 1/4 L 1/2 1 = 0 Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 33 / 51

34 Economic Example Continued Dividing the first first-order condition by the second first-order condition gives us (1/2)L/K = 2, or, L = 4K Substituting this into the second first-order condition, we get 5K 1/4 (4K ) 1/2 = 1, or K 1/4 = 2/5, or, Finally, we have, K = (5/2) 4 = 625/16 L = 625/4 This is our candidate solution for a maximum. But to verify that it really is the maximum we need to check the second-order conditions. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 34 / 51

35 Economic Example Continued The candidate solutions are, K = 625/16 and L = 625/4. The Hessian matrix at any (K, L) is given by [ 15 H = 8 K 7/4 L 1/2 5 4 K 3/4 L 1/2 ] 5 4 K 3/4 L 1/2 5 2 K 1/4 L 3/2 Note that since K > 0, L > 0. f 11 = 15 8 K 7/4 L 1/2 < 0 ( H = 5 2 ) 15 8 K 7/4+1/4 L 3/2+1/2 = (75/16)K 3/2 L 1 (25/16)K 3/2 L 1 = (50/16)K 3/2 L 1 > 0 ( ) 5 2 K 3/4 3/4 L 1/2 1/2 4 This shows that the second order conditions are satisfied at (K, L ), which shows it is a local maximum. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 35 / 51

36 Implicit Functions and Comparative Statics So far, we have consider the interpretation of functions with more than one variable through the Jacobian and the Hessian. We have also related the Hessian to the notions of concavity and convexity and maxima and minima, via an optimization problem. However, when we deal with functions of more than one variable, we also run into other complications. Basically, we tend to think of the LHS variable of a function as endogenous and the RHS variable(s) as exogenous. But sometimes exogenous variables and endogenous variables cannot be separated and we need to differentiate implicitly. Implicit differentiation is also useful when thinking about utility functions (indifference curves) and production functions (isoquant curves). Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 36 / 51

37 Implicit Function Theorem An example of an implicit function, i.e., one where the exogenous and endogenous variables cannot be separated, is, F (x, y) = x 2 + xye y + y 2 x 10 = 0 It is not clear how to separate out y from x or even whether it can be done. However, we may still want to find out how the endogenous variable changes when the exogenous variable changes. Under certain circumstances, it turns out that even when we cannot separate out the variables, we can nonetheless find the derivative dy/dx. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 37 / 51

38 Implicit Function Theorem The following result enables us to find the derivatives for implicit functions. Theorem: Let f (x, y) = 0 be an implicit function which is continuously differentiable and (x 0, y 0 ) be such that f (x 0, y 0 ) = 0. Suppose that f y (x 0, y 0 ) = 0. Then, dy dx f x (x0,y 0 ) = (x 0, y 0 ) f y (x 0, y 0 ) So, without specifying x and y we can find dy dx (x 0, y 0 ). at a given point, here Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 38 / 51

39 Abstract Example of Implicit Function Theorem Consider f (x, y) = x 2 + xye y + y 2 x 10 = 0. Suppose we want to evaluate the derivative at some point (x 0, y 0 ) such that f (x 0, y 0 ) = 0. Using the rules of differentiation, f y = xey + xye y + 2xy This can be zero for some x and y but assume that the point (x 0, y 0 ) is not one of them. Now differentiate with respect to x, f x = 2x + yey + y 2 The implicit function theorem allows us to write the following. dy dx (x0,y 0 ) = 2x 0 + y 0 e y 0 + y 2 0 x 0 e y 0 + x0 y 0 e y 0 + 2x0 y 0 Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 39 / 51

40 Economic Example of Implicit Function Theorem One use of the implicit function theorem in Economics is to find the slopes of indifference curves and isoquants. Consider the production function used earlier. y = K 0.25 L 0.75 If we fix y = y 0, then the combination of all (K, L) which gives y 0 units of output is called an isoquant. Suppose (K 0, L 0 ) is one such combination. We want to find the slope of the isoquant, that is dk dl (K0,L 0 ). Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 40 / 51

41 Economic Example Continued Setting y = y 0, we can write the production function in implicit function form as, F (K, L) = K 0.25 L 0.75 y 0 = 0 Using the values for the partial derivatives found earlier, we have, dk dl (K0,L 0 ) = Note that K 0 = 0, L 0 = K L K L = 3 K 0 L 0 This equation tells us the slope of the isoquant at (K 0, L 0 ). Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 41 / 51

42 Indifference Curves As a final example, consider the following utility function: U = [ αca η + βc η ] 1/η, b where a =apples, b =bananas. This is called a constant elasticity of substitution utility function. When η = 0 we get back to the Cobb-Douglas utility function. Holding utility fixed, we have the following: [ η αca + βc η ] 1/η b U0 = 0. Now use the implicit function theorem. dc U a (ca,0 c,c dc b,0 ) = a (c a,0, c b,0 ) U b c b (c a,0, c b,0 ) = α β ( ) η 1 ca Suppose we look at the MRS at the point (tc a, tc b ). The indifference curves have the same slope at (c a, c b ) and at (tc a, tc b ) for any t > 0. The CES utility function is an example of preferences that are homothetic. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 42 / 51 c b

43 Comparative Statics and Implicit Relations: Supply and Demand One question we also often need to answer is how underlying parameters affect the equilibrium of a supply and demand system. Suppose that we have a market where the demand and supply functions are given by the following. D = a bp + cy, S = α + βp The parameters a, b, c, α, β are all positive constants. The equilibrium price is, p = a α + cy b + β We can now check how the underlying parameters (a, b, c, y, α, β) affect the equilibrium price. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 43 / 51

44 Comparative Statics: Supply and Demand In our simple example, we can get the partial derivative, p / a = 1/(b + β) All other things remaining constant, an increase in a, increases the equilibrium price. We can similarly find the impact of changes in other parameters on the equilibrium price. Importantly, we can also, in many cases, conduct the comparative statics without solving for the equilibrium values of the endogenous variables. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 44 / 51

45 General Function Example of Supply and Demand Consider the following system more general demand and supply system. q d = D(p, T ), q s = S(p, T ) Here, T is the temperature on a given day. We assume that D p < 0, D T > 0, S p > 0, S T < 0. Equilibrium requires that D(p, T ) = S(p, T ) but we cannot compute the equilibrium price explicitly. Denote the equilibrium price p (T ): observe that it is a function of the parameter T. Inserting p (T ) into the equilibrium condition, we have, D(p (T ), T ) = S(p (T ), T ) Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 45 / 51

46 General Function Example of Supply and Demand We differentiate both sides with respect to T which gives, Rearranging, we get dp D p dt + D dp T = S p dt + S T dp dt = S T D T D p S p Note that since S T < 0, D T > 0, D p < 0, S p > 0 it follows that dp /dt > 0. Hence, even without being able to compute p (T ) explicitly, we can still say that the equilibrium price increases following an increase in the temperature. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 46 / 51

47 Example Continued Can we say anything about the equilibrium quantity? Note that the equilibrium quantity must satisfy, q (T ) = D(p (T ), T ) Differentiating with respect to T we get dq dt = D dp p dt + D T = D ps T S p D T D p S p In this case, the denominator is positive but we cannot say anything about the numerator unless we have more specific information about the magnitudes of D p, S p, D T and S T. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 47 / 51

48 General Formulation Suppose we have a general set of equilibrium conditions given by f 1 (x 1,..., x n ; α 1,..., α m ) = 0 f 2 (x 1,..., x n ; α 1,..., α m ) = 0... =... f n (x 1,..., x n ; α 1,..., α m ) = 0 Let us suppose that we can solve this set of equations to get (x1,..., x n ). Note that each xi will be a function of all the underlying parameters (α 1,..., α n ). In comparative statics, we typically are interested in knowing how (x 1,..., x n ) change when there is a small change in one of the parameters, say α i. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 48 / 51

49 General Formulation We proceed by differentiating each of the equilibrium conditions with respect to α i. This gives f1 1 dx1 + f 1 dx f 1 dxn n = fα 1 dα i dα i dα i i + f 2 dx f 2 dxn n = fα 2 dα i dα i dα i i f1 2 dx1. =. f1 n dx1 + f n dx f n dxn n = fα n dα i dα i dα i i Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 49 / 51

50 General Formulation In matrix notation this can be written as f1 1 f f 1 n dx f1 2 f fn 2 1 dα i dx dα i f1 n f2 n... fn n. dx n dα i How do we solve for dxj /dα i? Cramer s rule). = f 1 α i f 2 α i. f n α i Using the usual methods (like All of this gives us a very powerful tool more analyzing relatively complicated economics models in a simple way. Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 50 / 51

51 Roundup You should now be able to do/understand the following: 1 Partial differentiation and the Jacobian matrix 2 The Hessian matrix and concavity and convexity of a function 3 Optimization (profit maximization) 4 Implicit functions (indifference curves and comparative statics) Dudley Cooke (Trinity College Dublin) Multi-variable Calculus and Optimization 51 / 51

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