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STOCHASTIC

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PORTFOLIO CHOICE AND THE KELLY CRITERION<br />

counterexamples including the bounded utilities x y /y, y < 0. Also the counterexamples<br />

satisfying the hypotheses of our theorem include many bounded<br />

utilities [e.g., U(x) = tan -1 x, x>0, and V(x) = 1 -e' x , x ^ 0].<br />

III. An Outline of the Theory of Logarithmic Utility<br />

The simplest case is that of Bernoulli trials with probability p of success,<br />

0 < p < 1. The unique strategy which maximizes E log X„ is to bet at trial n<br />

the fixed fraction/* = p—q of total current wealth Xn_u if p > \ (i.e., if the<br />

expectation is positive), and to bet nothing otherwise.<br />

To maximize E log X„ is equivalent to maximizing E \og\_XJXo] 1 '" = (?(/),<br />

which we call the (exponential) rate of growth (per time period). It turns out<br />

that for p > \, G{f) has a unique positive maximum at/* and that there is<br />

a critical fraction/, 0

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