Topic 8: Optimisation of functions of several variables
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Transcript of Topic 8: Optimisation of functions of several variables
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Topic 8: Topic 8: Optimisation of functions Optimisation of functions
of several variablesof several variables
Topic 8: Topic 8: Optimisation of functions Optimisation of functions
of several variablesof several variablesUnconstrained OptimisationUnconstrained Optimisation
(Maximisation and Minimisation)(Maximisation and Minimisation)
Jacques (4th Edition): 5.4Jacques (4th Edition): 5.4
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Recall……Max
Min
X
Y
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First-Order / Necessary Condition:
dY/dX = f (X) = 0 Second-Order / Sufficent Condition:
d2Y/dX2 = f (X) if > 0 (Min) if < 0 (Max)
Max Y = f (X) X*
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Re-writing in terms of total differentials….
Max Y = f (X) X* Necessary Condition:
dY = f (X).dX = 0 , so it must be that f (X)= 0 Sufficent Condition:
d2Y = f (X).dX2 >0 for Min <0 for Max For Positive Definite, (Min ), it must be that f > 0 Negative Definite, (Max), it must be that f < 0
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Z
Y
X 0
A At A, dY = 0 And d2Y <0
Z
X
Y
0
B
At B, dY = 0 And d2Y >0
Max Y = f(X,Z) [X*Z*]
Min Y = f(X,Z) [X*Z*]
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Max Y = f (X, Z)[X*, Z*]
Necessary Condition:dY = fX.dX + fZ.dZ = 0so it must be that fX = 0 AND fZ = 0
Sufficient Condition:
d2Y= fXX.dX2 +fZX dZ.dX + fZZ.dZ2 + fXZ .dXdZ
….and since fZX = fXZ
d2Y= fXX.dX2 + fZZ.dZ2 + 2fXZ dX.dZ ?
>0 for Min<0 for Max
Sign Positive Definite Min Sign Negative Definite Max
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d2Y= fXX.dX2 + fZZ.dZ2 + 2fXZ dX.dZ
Complete the Square
222
2 dZf
fffdZ
f
fdXfYd
XX
XZZZXX
XX
XZXX
Sign Positive Definite To ensure d2Y > 0 and Min:
00 ZXXZZZXXXX ffffandf Sign Negative Definite To ensure d2Y < 0 and Max:
00 ZXXZZZXXXX ffffandf note: fXZ.fZX = (fXZ)2
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Optimisation - A summing Up… Condition Y = f(X) Y = f(X,Z)
Neccesary
So required that…….
dY = 0
fX = 0
dY = 0
fX = 0 AND fZ = 0
Sufficient For Min
So required that …..
d2Y > 0
fXX >0
d2Y > 0
fXX > 0 AND fXX fZZ – (fXZ)2 >0
Sufficient For Max
So required that …..
d2Y < 0
fXX < 0
d2Y < 0
fXX < 0 AND fXX fZZ – (fXZ)2 >0
fXX fZZ – (fXZ)2 <0 Saddle Point
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ExamplesFind all the maximum and minimum values of the functions:
(i) xzzzxxy 21622010 22
Necessary Condition for max or min: 1. 02420 zxfx and
2. 02216 xzf z Solve simultaneously
2042 xz
1622 xz
So 162204 xx 2x
Subbing in x = 2 to eq. 1 (or 2): 122 z and
6z
There is 1 stationary point at (2,6)
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S e c o n d O r d e r C o n d i t i o n s : 04 xxf
02 zzf
2xzf
T h u s , f X X f Z Z – ( f X Z ) 2 = ( - 4 . - 2 ) – ( - 2 ) 2 = + 8 – 4 = + 4 > 0 S u f f i c i e n t c o n d i t i o n f o r M a x ( d 2 Y < 0 ) . f x x < 0 a n d f X X f Z Z – ( f X Z ) 2 > 0 S o , M a x a t ( 2 , 6 )
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Example 2( i i ) xzzzxxy 85945100 22 N e c e s s a r y C o n d i t i o n f o r m a x o r m i n :
1 . 0885 zxf x a n d 2 . 08109 xzf z S o l v e s i m u l t a n e o u s l y
zx 858 zx 1098
zz 10985 42 z
2z S u b b i n g i n z = 2 t o e q 1 ( o r 2 )
112858 x 811x T h e r e i s 1 s t a t i o n a r y p o i n t a t 2,811
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Second Order Conditions 08 xxf
010 zzf
8xzf fXX fZZ – (fXZ)2 =
(8.10) – (82) = 80 – 64 = 16 ….>0 Sufficient condition for Min(d2Y > 0). fxx > 0 and fXX fZZ – (fXZ)2 >0
So, Min at 2,811
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Example 3(iii) zz
xxy 52
220 2
2
Necessary Condition for max or min:
1. 020 xf x and
2. 054 zf z Thus, 20x and 45z There is 1 stationary point at 45,20 Second Order Conditions
01xxf 04 zzf
0xzf fXX fZZ – (fXZ)2 = (-1.4) – (02) = - 4 < 0 Since, fxx < 0 and fXX fZZ – (fXZ)2 <0, Saddle Point at 45,20
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Optimisation of Optimisation of functions of several functions of several
variablesvariables
Optimisation of Optimisation of functions of several functions of several
variablesvariables
Economic ApplicationsEconomic Applications
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Example 1A firm can sell its product in two
countries, A and B, where demand in country A is given by PA = 100 – 2QA and in country B is PB = 100 – QB.
It’s total output is QA + QB, which it can produce at a cost of
TC = 50(QA+QB) + ½ (QA+QB)2
How much will it sell in the two countries assuming it maximises profits?
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Objective Function to Max is Profit….
= TR - TC = PAQA + PBQB – TC
PAQA = (100 – 2QA)QA
PBQB = (100 – QB)
QB
= 100QA – 2QA2 + 100QB – QB
2
– 50QA – 50QB – ½ (QA+QB)
2
= 50QA – 2QA2 + 50QB – QB
2 – ½
(QA+QB)2
Select QA and QB to max :
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if = 50QA – 2QA2 + 50QB – QB
2 – ½
(QA+QB)2
F.O.C. d =0
QA =50 - 4QA – ½ *2 (QA+QB) = 0
= 50 - 5QA – QB = 0 (1)
QB = 50 - 2QB – ½ *2 (QA+QB) = 0
= 50 - 3QB – QA = 0 (2)
50 - 5QA – QB = 50 - 3QB – QA
2QA = QB
Thus, output at stationary point is (QA,QB) = (71/7, 14 2/7 )
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Check Sufficient conditions for Max: d2 <0
QA = 50 - 5QA – QB
QB = 50 - 3QB – QA
Then QAQA = – 5 < 0
QAQA. QBQB – (QAQB)2 >0
(–5 * –3)) – (-1) 2 = 14 > 0 Max So firm max profits by selling 71/7 units to
country A and 14 2/7 units to country B.
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Example 2 Profits and production
Max = PQ(L, K) – wL - rK
{L*, K*}
Total Revenue = PQ
Expenditure on labour L = wL
Expenditure on Capital K = rK
Find the values of L & K that max
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Necessary Condition: d = 0
L = PQL – w = 0 , MPL = QL = w/P
K = PQK – r = 0 , MPK = QK = r/P
Sufficient Condition for a max, d2 <0
So LL < 0 AND (LL.KK - LK.KL) > 0
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Max = 2 K1/3L1/2 – L – 1/3 K{L*, K*}
Necessary condition for Max: d =0
(1) L = K1/3L-1/2 – 1 = 0
(2) K = 2/3 K-2/3 L1/2 – 1/3 = 0
Stationary point at [L*, K*] = [4, 8] note: to solve, from eq1: L½ = K1/3 . Substituting into eq2 then, 2/3K
– 2/3K1/3 = 1/3. Re-arranging K– 1/3 = ½ and so K 1/3 = 2 = L½.
Thus, K* =23= 8. And so L* = 22 = 4.
NOW, let Q = K1/3L1/2, P = 2, w = 1, r =1/3
Find the values of L & K that max ?
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L = K1/3L-1/2 – 1
K = 2/3 K-2/3 L1/2 – 1/3
LL = -1/2K
1/3L-3/2 < 0 for all K and L
KK = – 4/9 K–5/3L½
KL = LK = 1/3K–2/3L-½
For sufficient condition for a max,
Check d2 <0; LL < 0 & (LL.KK - LK.KL)>0
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LL.KK =(-1/2K1/3L-3/2 ).( – 4/9 K
–5/3L½ )
= 4/18 . K–4/3L-1
KL2 = (1/3K
–2/3L-½). (1/3K–2/3L-½)
= 1/9K–4/3L-1
Thus, LL.KK >
KL.LK since 4/18 > 1/9
So, (LL.KK - KL.LK) >0 for all values of K
& L Profit max at stationary point [L*, K*] = [4, 8]
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Unconstrained Optimisation –
Functions of Several Variables
• Self-Assessment Questions on Website
• Tutorial problem sheets • Pass Exam Papers• Examples in the Textbook