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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
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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 zi is a linear function of some random factor fi and an idiosyncratic term i.
Abstracting away from the idiosyncratic risk • If the bis of two assets are the same, then the ais must be the same for an arbitrage free model. • Suppose we are interested in forming a portfolio of two assets with different bis, bi bj , bi 0, bj 0
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
The matrix must be singular, or the first row must be a linear combination of the last two rows