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Introduction to Computational Finance and
Financial Econometrics
Portfolio Theory with Matrix Algebra
Eric Zivot
Spring 2015
Eric Zivot (Copyright©2015)Portfolio Theory1 / 54
Outline
1Portfolios with Three Risky Assets
Portfolio characteristics using matrix notation
Finding the global minimum variance portfolio
Finding efficient portfolios
Computing the efficient frontier
Mutual fund separation theorem again
Eric Zivot (Copyright©2015)Portfolio Theory2 / 54
Example
Example: Three risky assets
LetRi(i=A,B,C) denote the return on assetiand assume thatRi follows CER model: R i≂iid N(μi,σ2i) cov(Ri,Rj) =σij
Portfolio "x":
x i= share of wealth in asseti x
A+xB+xC= 1
Portfolio return:
R p,x=xARA+xBRB+xCRC.Eric Zivot (Copyright©2015)Portfolio Theory3 / 54
Example cont.
Stocki μiσiPair (i,j)σijA (Microsoft) 0.0427 0.1000 (A,B) 0.0018
B (Nordstrom) 0.0015 0.1044 (A,C) 0.0011
C (Starbucks) 0.0285 0.1411 (B,C) 0.0026Three asset example data.
In matrix algebra, we have:
A B C) (0.0427
0.0015
0.0285)
2AσABσAC
ABσ2BσBC
ACσBCσ2C)
((0.1000)20.0018 0.0011
0.0018 (0.1044)20.0026
0.0011 0.0026 (0.1411)2)
)Eric Zivot (Copyright©2015)Portfolio Theory4 / 54
Outline
1Portfolios with Three Risky Assets
Portfolio characteristics using matrix notation
Finding the global minimum variance portfolio
Finding efficient portfolios
Computing the efficient frontier
Mutual fund separation theorem again
Eric Zivot (Copyright©2015)Portfolio Theory5 / 54
Matrix Algebra Representation
R=( (R A R B R C) A B C) ),1=( (1 1 1) x=( (x A x B x C)
2AσABσAC
ABσ2BσBC
ACσBCσ2C)
Portfolio weights sum to 1:
x ?1= (xAxBxC)( (1 1 1) =x1+x2+x3= 1Eric Zivot (Copyright©2015)Portfolio Theory6 / 54
Portfolio return
R p,x=x?R= (xAxBxC)( (R A R B R C) =xARA+xBRB+xCRC
Portfolio expected return:
p,x=x?μ= (xAxBxX)( A B C) =xAμA+xBμB+xCμCEric Zivot (Copyright©2015)Portfolio Theory7 / 54
Computational tools
R formula:
t(x.vec)%*%mu.vec crossprod(x.vec, mu.vec)
Excel formula:
MMULT(transpose(xvec),muvec)
--Eric Zivot (Copyright©2015)Portfolio Theory8 / 54 Portfolio variance
2p,x=x?Σx
xAxBxC)( 2AσABσAC
ABσ2BσBC
ACσBCσ2C)
(x A x B x C) =x2Aσ2A+x2Bσ2B+x2Cσ2C + 2xAxBσAB+ 2xAxCσAC+ 2xBxCσBC Portfolio distribution:
R p,x≂N(μp,x,σ2p,x)Eric Zivot (Copyright©2015)Portfolio Theory9 / 54 Computational tools
R formulas:
t(x.vec)%*%sigma.mat%*%x.vec Excel formulas:
MMULT(TRANSPOSE(xvec),MMULT(sigma,xvec))
MMULT(MMULT(TRANSPOSE(xvec),sigma),xvec)
--Eric Zivot (Copyright©2015)Portfolio Theory10 / 54 Covariance Between 2 Portfolio Returns
2 portfolios:
x=( (x A x B x C) ),y=( (y A y B y C) x ?1= 1,y?1= 1 Portfolio returns:
R p,x=x?R R p,y=y?R Covariance:
cov(Rp,x,Rp,y) =x?Σy =y?ΣxEric Zivot (Copyright©2015)Portfolio Theory11 / 54 Computational tools
R formula:
t(x.vec)%*%sigma.mat%*%y.vec Excel formula:
MMULT(TRANSPOSE(xvec),MMULT(sigma,yvec))
MMULT(TRANSPOSE(yvec),MMULT(sigma,xvec))
--Eric Zivot (Copyright©2015)Portfolio Theory12 / 54 Derivatives of Simple Matrix Functions
LetAbe ann×nsymmetric matrix, and letxandybe ann×1 vectors. Then, ∂∂xn×1x ?y=( ((∂∂x 1x?y ∂∂x nx?y) ))=y,(1) ∂∂xn×1x ?Ax=( ((∂∂x 1x?Ax ∂∂x nx?Ax) ))= 2Ax.(2)Eric Zivot (Copyright©2015)Portfolio Theory13 / 54 Outline
1Portfolios with Three Risky Assets
Portfolio characteristics using matrix notation
Finding the global minimum variance portfolio
Finding efficient portfolios
Computing the efficient frontier
Mutual fund separation theorem again
Eric Zivot (Copyright©2015)Portfolio Theory14 / 54 Computing Global Minimum Variance Portfolio
Problem: Find the portfoliom= (mA,mB,mC)?that solves: min mA,mB,mCσ2p,m=m?Σms.t.m?1= 11Analytic solution using matrix algebra 2Numerical Solution in Excel Using the Solver (see
3firmExample.xls)
Eric Zivot (Copyright©2015)Portfolio Theory15 / 54 Analytic solution using matrix algebra
The Lagrangian is:
L(m,λ) =m?Σm+λ(m?1-1)
First order conditions (use matrix derivative results): 0 (3×1)=∂L(m,λ)∂m=∂m?Σm∂m+∂∂mλ(m?1-1) = 2·Σm+λ1 0 (1×1)=∂L(m,λ)∂λquotesdbs_dbs17.pdfusesText_23