Basic Mathematics for Economists phần 9 ppsx

Given the demand and supply schedulesQdt = 3450 − 6P t and Qst = −729 + 4.5P t−1 use difference equations to predict what price will be in the tenth time period after an unexpected drop

Now substitute this value for P0into the general solution (2) above, so that

Price therefore continually fluctuates between 35 and 55

This is the third possibility in the stability conditions examined earlier In this example

Test Yourself, Exercise 13.2

(Assume that the usual cobweb assumptions apply in these questions.)

3 Given the demand and supply schedules

Qdt = 3450 − 6P t and Qst = −729 + 4.5P t−1

use difference equations to predict what price will be in the tenth time period after

an unexpected drop in quantity to 354, assuming that the market was previously

in long-run equilibrium

13.4 The lagged Keynesian macroeconomic model

In the basic Keynesian model of the determination of national income, if foreign trade andgovernment taxation and expenditure are excluded, the model reduces to the accountingidentity,

If there is a disturbance from this equilibrium, e.g exogenous investment I alters, then the

adjustment to a new equilibrium will not be instantaneous This is the basis of the well-knownmultiplier effect An initial injection of expenditure will become income for another sector

of the economy A proportion of this will be passed on as a further round of expenditure, and

so on until the ‘ripple effect’ dies away

Because consumer expenditure may not adjust instantaneously to new levels of income,

a lagged effect may be introduced If it is assumed that consumers’ expenditure in onetime period depends on the income that they received in the previous time period, then theconsumption function becomes

where the subscripts denote the time period

National income, however, will still be determined by the sum of all expenditure withinthe current time period Therefore the accounting identity (1), when time subscripts areintroduced, can be written as

From (3) and (4) we can derive a difference equation that explains how Y t depends on Y t−1.Substituting (3) into (4) we get

The level of investment I tthen falls to 110 and remains at this level each time period Trace

out the pattern of adjustment to the new equilibrium value of Y , assuming that the model was

initially in equilibrium

Solution

Although this pattern of adjustment can best be viewed using a spreadsheet, let us firstwork out the first few steps of the process manually and relate them to the familiar 45◦-lineincome-expenditure graph (illustrated inFigure 13.3) often used to show how Y is determined

in introductory economics texts

If the system is initially in equilibrium then income in one time period is equal toexpenditure in the previous time period, and income is the same each time period Thus

Y t = Y t−1= Y∗

where Y∗is the equilibrium level of Y Therefore, when the original value of I

t of 134 isinserted into the accounting identity the model becomes

This is the initial equilibrium value of Y before the change in I

Assume time period 0 is the one in which the drop in I to 110 occurs Consumption in time period 0 will be based on income earned the previous time period, i.e when Y was still

at the old equilibrium level of 435 Thus

Y3= C3+ I3

= (40 + 0.6Y2)+ 110

= 40 + 0.6(387.96) + 110

= 382.776

and so on It can be seen that in each time period Y decreases by smaller and smaller amounts

as it readjusts towards the new equilibrium value This new equilibrium value can easily becalculated using the same method as that used above to work out the initial equilibrium

When I = 110 and Y t = Y t−1= Y∗then the model becomes

to scale and just shows the direction and relative magnitude of the steps in the adjustmentprocess.)

Unlike the cobweb model described earlier, the adjustment in this Keynesian model isalways in the same direction, instead of alternating on either side of the final equilibrium

Successive values of Y just approach the equilibrium by smaller and smaller increments

because the ratio in the complementary function to the difference equation (explained below)

is not negative as it was in the cobweb model If the initial equilibrium had been below the

new equilibrium then, of course, Y would have approached its new equilibrium from below

instead of from above

Further steps in the adjustment of Y in this model are shown in the Excel spreadsheet in

Table 13.4, which is constructed as explained inTable 13.5 This clearly shows Y closing in

on its new equilibrium as time increases

Difference equation solution

Let us now return to the problem of how to solve the difference equation

The general solution can then be applied to numerical problems, such as Example 13.7 above

By ‘solving’ this difference equation we mean putting it in the format

Y t = f(t)

so that the value of Y t can be determined for any given value of t.

E5 134 Original given investment level.

E6 110 New investment level

D6 160 This is initial “shock” quantity in time period 0

A10 to

A28

Enter numbers from 0 to 18

These are the time periods used

E7 =(B5+E5)/(1-B6) Calculates initial equilibrium value of Y using

formula Y = (a + I)/(1 - b).

E8 =(B5+E6)/(1-B6) Same formula calculates new equilibrium

value of Y, using new value of I in cell E6

B10 =B5+B6*E7 Calculates consumption in time period 0 using

formula C = a + bY t – 1 where Y t – 1 is the old equilibrium value in cell E7

C10 =B10+E$6 Calculates Y in time period 0 as sum of

current consumption value in cell B10 and new investment value Note the $ on cell E6 toanchor when copied

B11 =B$5+B$6*C10 Calculates consumption in time period 1 based

on Y0 value in cell C10 Note the $ on cells B5and B6 to anchor when copied

If the given values of a, b and I t are put into (4) then the equilibrium value of Y is determined.

This is the first part of the difference equation solution

Returning to the difference equation (1) which we are trying to solve

where A and k are unknown parameters This means that

Y t = a + I t

If t is increased, then the value of b t in the general solution (9) will diminish as long as

|b| < 1 This condition will be met since b is the marginal propensity to consume which has been estimated to lie between 0 and 1 in empirical studies Therefore Y t will always headtowards its new equilibrium value

The value of the constant A can be determined if an initial value Y0is known Substitutinginto (9), this gives

Thus A is the value of the difference between the initial level of income Y0, immediately

after the shock, and its final equilibrium value Y∗.

Putting this result into (9) above, the general solution to our difference equation becomes

We can now check that this solution to the lagged Keynesian model difference equationworks with the numerical Example 13.7 considered above This model assumed

Y t = C t + I t

where I twas initially 134 and

C t = 40 + 0.6Y t−1

which corresponded to an initial equilibrium of Y tof 435

When I twas exogenously decreased to 110, the adjustment path towards the new

equilib-rium value of Y of 375 was worked out by an iterative method Now let us see what values

our difference equation will give

We have to be careful in determining the initial value Y0, immediately after the increase

in investment has taken place This depends on I0, which will be the new level of investment

of 110, and C0 The level of consumption in period 0 depends on the previously existing

equilibrium level of Y t which was 435 in time period ‘minus one’ Therefore

This difference equation solution can now be used to calculate Y tin any given time period.For example, in time period 9 it will be

Y9= 375 + 36(0.6)9= 375 + 0.3628 = 375.3628

As t increases in value, eventually the value of (0.6) becomes so small as to make the second

term negligible In the above example we can say that for all intents and purposes Y thas tively reached its equilibrium value of 375 by the ninth time period, although theoretically

effec-Y twould never actually reach 375 if infinitesimally small increments were allowed

By now, many of you may be thinking that this difference equation method of computing

the different values of Y in the adjustment process in a Keynesian macroeconomic model is

extremely long-winded and it would be much quicker to compute the values by the iterativemethod, particularly if a spreadsheet can be used

In many cases you may be right However, you must remember that this chapter is onlyintended to give you an insight into the methods that can be used to trace out the timepath of adjustment in dynamic economic models The mathematical methods of solutionexplained here can be adapted to tackle more complex problems that cannot be illustrated

on a spreadsheet Also, economists need to set up mathematical formulations for functionalrelationships in order to estimate the parameters of these functions Those of you who studyeconometrics after the first year of your course will discover that the algebraic solutions todifference equations can help in the setting up of models for testing certain dynamic economicrelationships

Now that the general solution to the lagged Keynesian macroeconomic model has beenderived, it can be applied to other numerical examples and may even allow you to computeanswers more quickly than by switching on your computer and setting up a spreadsheet

Example 13.8

There is initially an equilibrium in the basic Keynesian model

Y t = C t + I t

C t = 650 + 0.5Y t−1

with I t remaining at 300 Then I tsuddenly increases to 420 and remains there What will be

the actual level of Y six time periods after this change?

Substituting these values into the general solution for the lagged Keynesian macroeconomicmodel difference equation, we get the general solution for this example, which is

Therefore Y will have exceeded 2,130 by the end of the fourth time period.

Only the most basic lagged Keynesian model has been considered so far in this section.Other possible formulations have been suggested for the ways in which past income levelscan determine current expenditure For example

C t = a + bY t−2

C t = a + b1Y t−1+ b2Y t−2

The latter example is known as a ‘distributed lag’ model The solutions of these more plex models require more advanced mathematical methods than are explained in this basicmathematics text You should, however, be able to adapt the spreadsheet set up inTable 13.4

com-to trace out the adjustment path of income in a distributed lag model with given parameters

Example 13.10

Use a spreadsheet to estimate Y t for the twelve time periods after I t is increased to 140,

assuming that Y t is determined by the distributed lag Keynesian model

Y t = C t + I t

C t = 320 + 0.5Y t−1+ 0.3Y t−2

and that the system had previously been in equilibrium with I at 90.

Solution

This is a spreadsheet exercise that you can do yourself by making the necessary adjustments

to the formulae that were used to set up the spreadsheet in Table 13.4 when tackling

Exam-ple 13.7 Be careful in setting up the initial values, however, as C will depend on the old equilibrium level of Y up to period 1 Some of the initial values are calculated manually

below for you to check against

The initial equilibrium level Y∗would have satisfied the equations

Test Yourself, Exercise 13.3

1 A Keynesian macroeconomic model with a single-time-period lag on the sumption function, as described below, is initially in equilibrium with the level of

con-I t given at 500

Y t = C t + I t

C t = 750 + 0.5Y t−1

I t is then increased to 650 Use difference equation analysis to find the value of

Y t in the fourth time period after this disturbance to the system Will it then bewithin 1% of its new equilibrium level?

2 There is initially an equilibrium in the macroeconomic model

Y t = C t + I t

C t = 2,500 + 0.9 Y t−1

with the level of I t set at 1,100 Investment is then increased to 1,500 where it

remains for future time periods Calculate what the level of Y twill be in the fortiethtime period after this investment increase

3 In a basic Keynesian model with a government sector

4 Use a spreadsheet to trace out the pattern of adjustment of Y t towards its newequilibrium value in the model

Y t = C t + I t

C t = 310 + 0.7Y t−1

if I t is exogenously increased from 240 to 350 and then kept at this new level

Assume the system was initially in equilibrium What is the value of C in the fourteenth time period after this increase in I t?

13.5Duopoly price adjustment

An oligopoly is a market with a small number of sellers It is difficult for economists topredict price and output in oligopoly because firms’ reactions to their rivals’ actions can varydepending on the strategy they adopt Firms may naively assume that rivals will not react

to whatever pricing policy they themselves operate, they may try to outguess their rivals,

or they may collude You will learn more about these different models in your economicscourse Here we will just examine how price may adjust over time in one of the simpler

‘naive’ models applied to a duopoly, which is an oligopoly with only two sellers

Two models, the Cournot model and the Bertrand model, assume that firms do not thinkahead The Cournot model assumes that firms think that their rivals will not change theiroutput in response to their own output decisions and the Bertrand model assumes that firmsthink that rivals will not change their price

Without going into the details of the model, the predictions of the Bertrand model can besummarized in terms of the ‘reactions functions’ shown in Figure 13.4 for two duopolists Xand Y These show the price that will maximize one firm’s profits given the value of the other

firm’s price read off the other axis For example, X’s reaction function RX slopes up fromright to left If Y’s price is higher, then X can get away with a higher price, but if Y lowersits price then X also has to reduce its price otherwise it will lose sales

This model assumes that both firms have identical cost structures and so the reactionfunctions are symmetrical, intersecting where they both cross the 45◦-line representing equal

prices The prediction is that prices will eventually settle at levels P X

∗ and P∗Y, which are

equal The path of adjustment from an initial price P X

0 is shown in Figure 13.4 In time

period 1, Y reacts to P0X by setting price P1Y ; then X sets price P2Xin time period 2, and so

on until P X

∗ and P∗Y are reached Note that we are assuming that the firms take it in turns

to adjust price and so each firm only sets a new price every other time period More recentapplications of this basic model based on Game Theory assume that forms go straight to theintersection point, which is called the ‘Nash equilibrium’

Let us now use our knowledge of difference equations to derive a function that will tell uswhat the price of one of the firms will be in any given time period with the aid of a numericalexample

If the assumptions of the Bertrand model hold, derive a difference equation for P t X and

calculate what P t Xwill be in time period 10 if firm X starts off in time period 0 by setting aprice of 300

Note that the value P t X−2 appears because we have substituted in the reaction function of Y

for period t − 1 to correspond to the value of P Y

t−1in X’s reaction function, i.e X is reacting

to what Y did in the previous time period which, in turn, depends on what X did in the periodbefore that one and so we have substituted in the equation

P t Y−1= 45 + 0.8P X

t−2The particular solution to the difference equation (3) will be where price no longer changes and

where the constant 81 is ignored, gives

To find the value of A we need to know the specific value of P t Xat some point in time The

question specifies that initially (i.e in time period 0) P t Xis 300 and so

P0X = 300 = A(0.8)0+ 225

300= A + 225

A= 75

Thus, as in the previous difference equation applications, A is the difference between the

final equilibrium and the initial value of the variable in question The general solution to thedifference equation (1) is therefore

P t X = 75(0.8) t+ 225

This can be used to calculate the value of P X

t every alternate time period (Remember that

in the intervening time periods X keeps price constant while Y adjusts price.) The priceadjustment path will therefore be

Example 13.12

Two firms X and Y in an oligopolistic market take a shortsighted view of their situation andset price on the basis of their rivals’ price in the previous time period according to the reactionfunctions

P t X = 300 + 0.75P Y

t−1

P t Y = 300 + 0.75P X

t−1Assume that each adjusts its price every other time period The market is initially in equilib-

rium with P X

t = P Y

t = 1,200 Firm X then decides to try to improve its profits by raising

price to 1,650 Taking into account the reactions to rivals’ price changes described in theabove functions, calculate what X’s price will be in the eighth time period after its breakawayprice rise

Adding the complementary function and particular solution, the general solution to thedifference equation becomes

Test Yourself, Exercise 13.4

1 Two duopolists X and Y react to each others’ prices according to the functions

equilibrium level, but how short-lived? Calculate whether or not PXwill be backwithin 1% of its equilibrium value within six time periods (Be careful how you

calculate the value of A in the difference equation as this time the initial value is below the equilibrium value of PX.)

3 In a duopoly where the assumptions of the Bertrand model hold, the two firms’reaction functions are

P t X = 95.54 + 0.83P Y

t−1

P t Y = 95.54 + 0.83P X

t−1

If firm X unexpectedly changes price to 499, derive the solution to the difference

equation that determines P t X and use it to predict P t Xin the twelfth time periodafter the initial change

14 Exponential functions, continuous

growth and differential equations

Learning objectives

After completing this chapter students should be able to:

• Use the exponential function and natural logarithms to derive the final sum, initialsum and growth rate when continuous growth takes place

• Compare and contrast continuous and discrete growth rates

• Set up and solve linear first-order differential equations

• Use differential equation solutions to predict values in basic market and economic models

macro-• Comment on the stability of economic models where growth is continuous

14.1 Continuous growth and the exponential function

InChapter 7, growth was treated as a process taking place at discrete time intervals In thischapter we shall analyse growth as a continuous process, but it is first necessary to understandthe concepts of exponential functions and natural logarithms The term ‘exponential function’

is usually used to describe the specific natural exponential function explained below However,

it can also be used to describe any function in the format

y = Ax where A is a constant and A > 1

This is known as an exponential function to base A When x increases in value this function obviously increases in value very rapidly if A is a number substantially greater than 1 On the other hand, the value of A x approaches zero if x takes on larger and larger negative values For all values of A it can be deduced from the general rules for exponents (explained in

Chapter 2) that A0= 1 and A1= A.

Example 14.1

Find the values of y = A x when A is 2 and x takes the following values:

(a) 0.5, (b) 1, (c) 3, (d) 10, (e) 0 , (f)−0.5, (g) −1, and (h) −3

( g) A−1= 0.50 ( h) A−3= 0.13

The natural exponential function

In mathematics there is a special number which when used as a base for an exponentialfunction yields several useful results This number is

2.7182818 (to 7 dp)

and is usually represented by the letter ‘e’ You should be able to get this number on yourcalculator by entering 1 and then using the [ex] function key

To find ex for any value of x on a calculator the usual procedure is to enter the number (x)

and then press the [ex] function key To check that you can do this, try using your calculator

to obtain the following exponential values:

In economics, exponential functions to the base e are particularly useful for analysinggrowth rates This number, e, is also used as a base for natural logarithms, explained later

in Section 14.4 Although it has already been pointed out that, strictly speaking, the specific

function y = ex should be known as the ‘natural exponential function’, from now on weshall adopt the usual convention and refer to it simply as the ‘exponential function’

To understand how this rather awkward value for e is derived, we return to the methodused for calculating the value of an investment developed inChapter 7 You will recall that

the final value (F ) of an initial investment (A) deposited for t discrete time periods at an interest rate of i can be calculated from the formula

F = A(1 + i) t

If the interest rate is 100% then i= 1 and the formula becomes

F = A(1 + 1) t = A(2) t

Assume the initial sum invested A= 1 If interest is paid at the end of each year, then after

1 year the final sum will be

F1= (1 + 1)1= 2

InChapter 7it was also explained how interest paid monthly at the annual rate divided by 12will give a larger final return than this nominal annual rate because the interest credited each

month will be reinvested When the nominal annual rate of interest is 100% (i= 1) and theinitial sum invested is assumed to be 1, the final sum after 12 months invested at a monthlyinterest rate of 121(100%) will be

This result means that a sum A invested for one year at a nominal annual interest rate of

100% credited continuously will accumulate to the final sum of

to go to the 4th decimal place to find a difference between the two.) Continuous growthalso occurs in other variables relevant to economics, e.g population, the amount of naturalmaterials mined Other variables may continuously decline in value over time, e.g the stock

of a non-renewable natural resource

14.2 Accumulated final values after continuous growth

To derive a formula that will give the final sum accumulated after a period of continuousgrowth, we first assume that growth occurs at several discrete time intervals throughout

a year We also assume that A is the initial sum, r is the nominal annual rate of growth, n is

the number of times per year that increments are accumulated and y is the final value Using

the final sum formula developed inChapter 7, this means that after t years of growth the final

Growth becomes continuous as the number of times per year that increments in growth are

accumulated increases towards infinity When n→ ∞ then n

The final value of the population (in millions) is found by using the formula y = Ae rt and

substituting the given numbers: initial value A = 4.5; rate of growth r = 3% = 0.03; number

of time periods t = 15, giving

y = 4.5e 0.03(15) = 4.5e 0.45 = 4.5 × 1.5683122 = 7.0574048 million

Thus the predicted final population is 7,057,405

Example 14.3

An economy is forecast to grow continuously at an annual rate of 2.5% If its GNP is currentlye56 billion, what will the forecast for GNP be at the end of the third quarter the year afternext?

Solution

In this example: t = 1.75 years, r = 2.5 % = 0.025, A = 56 (e billion) Therefore, the

final value of GNP will be

y = Ae rt = 56e0.025(1.75)= 56e0.04375 = 58.504384

Thus the forecast for GNP ise58,504,384,000.

So far we have only considered positive growth, but the exponential function can also

be used to analyse continuous decay if the rate of decline is treated as a negative rate ofgrowth

Therefore, the river flow will shrink to 14.16 million gallons per day

Continuous and discrete growth rates compared

In Section 14.1 it was explained how interest at a rate of 100% credited continuously

through-out a year gives an annual equivalent rate of r = e−1 = 1.7182818 = 171.83%, a difference

of 71.83% However, in practice interest is usually credited at much lower annual rates Thismeans that the difference between the nominal and annual equivalent rates when interest iscredited continuously will be much smaller This is illustrated inTable 14.1for the case whenthe nominal annual rate of interest is 6%

These figures show that the annual equivalent rate when interest is credited continuously isthe same as that when interest is credited on a daily basis, if rounded to two decimal places,although there will be a slight difference if this rounding does not take place

Test Yourself, Exercise 14.1

1 A country’s population is currently 32 million and is growing continuously at anannual rate of 3.5% What will the population be in 20 years’ time if this rate ofgrowth persists?

2 A company launched a successful new product last year The current weekly saleslevel is 56,000 units If sales are expected to grow continually at an annual rate

of 12.5%, what will be the expected level of sales 36 weeks from now? (Assumethat 1 year is exactly 52 weeks.)

3 Current stocks of mineral M are 250 million tonnes If these stocks are continuallybeing used up at an annual rate of 9%, what amount of M will remain after 30years?

4 A renewable natural resource R will allow an estimated maximum consumptionrate of 200 million units per annum Current annual usage is 65 million units

If the annual level of usage grows continually at an annual rate of 7.5% will there

be sufficient R to satisfy annual demand after (a) 5 years, (b) 10 years, (c) 15 years,(d) 20 years?

5 Stocks of resource R are shrinking continually at an annual rate of 8.5% Howmuch will remain in 30 years’ time if current stocks are 725,000 units?

6 Ife25,000 is deposited in an account where interest is credited on a daily basis

that can be approximated to the continuous accumulation of interest at a nominalannual rate of 4.5%, what will the final sum be after five years?

14.3 Continuous growth rates and initial amounts

Derivation of continuous rates of growth

The growth rate r can simply be read off from the exponent of a continuous growth function

in the format y = Ae rt

To prove that this is the growth rate we can use calculus to derive the rate of change of thisexponential growth function

If variable y changes over time according to the function y = Ae rt then rate of change of

y with respect to t will be the derivative dy/dt However, it is not a straightforward exercise

to differentiate this function For the time being let us accept the result (explained below in

Section 14.4) that

if y= et thendy

dt = et

i.e the derivative of an exponential function is the function itself

Thus, using the chain rule,

when y = Ae rt

then dy

dt = rAe rt This derivative approximates to the absolute amount by which y increases when there is a one unit increment in time t, but when analysing growth rates we are usually interested in the proportional increase in y with respect to its original value The rate of growth is therefore dy

dt

y =rAert

Aert = r

Even though r is the instantaneous rate of growth at any given moment in time, it must be

expressed with reference to a time interval, usually a year in economic applications, e.g 4.5%per annum It is rather like saying that the slope of a curve is, say, 1.78 at point X A slope of1.78 means that height increases by 1.78 units for every 1 unit increase along the horizontalaxis, but at a single point on a curve there is no actual movement along either axis

Example 14.5

Owing to continuous improvements in technology and efficiency in production, an empirical

study found a factory’s output of product Q at any moment in time to be determined by the

function

Q= 40e0.03t

where t is the number of years from the base year in the empirical study and Q is the output

per year in tonnes What is the annual growth rate of production?

Solution

When the accumulated amount from continuous growth is expressed by a function in the

format y = Ae rt then the growth rate r can simply be read off from the function Thus when

Q= 40e0.03t

the rate of growth is

r = 0.03 = 3%