The Arbitrage Pricing Model Lecture XXVI
A Single Factor Model Abstracting away from the specific form of the CAPM model, we posit a single factor model written as In this model, the random return on an investment z i is a linear function of some random factor f i and an idiosyncratic term i.
Abstracting away from the idiosyncratic risk If the b i s of two assets are the same, then the a i s must be the same for an arbitrage free model. Suppose we are interested in forming a portfolio of two assets with different b i s, b i b j, b i 0, b j 0
Computing the mean and variance of this portfolio yields
Holding the variance of the portfolio equal to zero, we find
Multifactor Models: Suppose that asset returns are generated by a two factor linear model: A portfolio of these assets then yields
Again to minimize systematic risk If the portfolio is riskless, then it yields zero profit
Given
The matrix must be singular, or the first row must be a linear combination of the last two rows