Sharpe portfolio using a cross-efficiency evaluation
  The Sharpe ratio is a way to compare the excess returns (over the risk free asset) of portfolios for each unit of volatility that is generated by a portfolio. In this paper we introduce a robust Sharpe ratio portfolio under the assumption that the risk free asset is unknown. We propose a robust portfolio that maximizes the Sharpe ratio when the risk free asset is unknown, but is within a given interval. To compute the best Sharpe ratio portfolio all the Sharpe ratios for any risk free asset are considered and compared by using the so-called cross-efficiency evaluation. An explicit expression of the Cross-Eficiency Sharpe ratio portfolio is presented when short selling is allowed. 
Juan F. Monge, Mercedes Landete and Jos\'e L. Ruiz
q-fin.PM