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Aug 30, 2018 at 15:42 vote accept Ivan
Aug 30, 2018 at 12:58 comment added Gordon Generally, you need another asset, such as the deposit account, to make the investment self-financing. But here we can only heuristically argue like that given that you have only the risky asset.
Aug 30, 2018 at 0:53 comment added Ivan thanks for your answer. I made the same reasoning. However, if you apply Ito's formula you get $dX_{t} = \dfrac{\alpha_{t}}{S_{t}} dS_{t} + S_{t} d\dfrac{\alpha_{t}}{S_{t}} + d \langle S_{t} , \dfrac{\alpha_{t}}{S_{t}} \rangle$, and then you need to have $ S_{t} d\dfrac{\alpha_{t}}{S_{t}} + d \langle S_{t} , \dfrac{\alpha_{t}}{S_{t}} \rangle = 0$ (to have the dynamics of the wealth process) what seems very convenient and do not have a financial sense.
Aug 29, 2018 at 14:10 history edited Gordon CC BY-SA 4.0
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Aug 29, 2018 at 13:06 history edited Gordon CC BY-SA 4.0
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Aug 29, 2018 at 13:00 history answered Gordon CC BY-SA 4.0