跟 leverage, volatility and time 都有关。。。

Minder Cheng and Ananth Madhavan, THE DYNAMICS OF LEVERAGED AND INVERSE EXCHANGE-TRADED FUNDS

Formula（27）has leverage \(x\) (could be negative for inverse), volatility \(\sigma\) and time \(t_N\) 。坛子里有金融教授出来解释一下吧。