各位朋友，能不能帮我下以下几篇文献，万分感谢！

16、

【篇名】The Theory of Hedgingand Speculation in Commodity Futures

【作者】Johnson,Leland L.

【网址】http://www.jstor.org/sici?sici=0034-6527(1960)27:3<139>2.0.CO;2-

17、

【篇名】A Capital AssetPricing Model with Time-varying Covariances

【作者】T Bollerslev, RFEngle, JM Wooldridge

【网址】http://www.jstor.org/stable/1830713

18、

【篇名】Hedging with stockindex futures: Estimation and forecasting with error correction model

【作者】Asim Ghosh

【网址】http://onlinelibrary.wiley.com/doi/10.1002/fut.3990130703/abstract

19、

【篇名】Estimatingtime-varying optimal hedge ratios on futures markets

【作者】Robert J. Myers

【网址】http://onlinelibrary.wiley.com/doi/10.1002/fut.3990110105/abstract

20、

【篇名】Comparison ofanalytical approaches for estimating hedge ratios for agricultural commodities

【作者】Harvey J. Witt;Ted C.Schroeder;Marvin L. Hayenga

【网址】http://onlinelibrary.wiley.com/doi/10.1002/fut.3990070204/abstract