Chapter 3 Matrix Algebra Review

Updated: April 27, 2021

This chapter reviews some basic matrix algebra concepts that we will use throughout the book. Many of the formulas that involve multiple variables that we derive in the book can be simplified using matrix algebra. This aids in understanding general concepts. For example, portfolio risk and return calculations for portfolios with more than two assets is easy to express using matrix algebra. If returns are jointly normally distributed, then this multivariate normal distribution can be expressed concisely using matrix algebra. The first order conditions involving the portfolio weights for mean-variance optimized portfolios is a system of linear equations that can be set-up and solved using matrix algebra. In addition, R is a matrix programming language. Pretty much anything that you can represent in matrix algebra you can evaluate in R using matrix calculations that have a syntax very much like the matrix algebra representation. Lastly, many R calculations can be efficiently evaluated if they are vectorized - that is, if they operate on vectors of elements instead of looping over individual elements.

This chapter is organized as follows. Section 3.1 gives definitions of matrices and vectors and shows how to represent these objects in R. Section 3.2 reviews basic matrix operations (addition, subtraction, multiplication), and section 3.3 shows how common summation calculations can be represented using matrix notation. Section 3.4 covers systems of linear equations and shows how the inverse of a matrix is related to the solutions to a system of linear equations. Section 3.5 discusses positive definite matrices and Cholesky decomposition. The use of matrix algebra for representing multivariate probability distributions is presented in section 3.6, and portfolio calculations using matrix algebra is presented in section 3.7. Section 3.8 concludes with a discussion of derivatives of some simple matrix functions.

The R packages used in this chapter are Matrix and mvtnorm. Make sure these packages are downloaded and installed prior to replicating the R examples in this chapter.