Portfolio selection in the case of no short sales of assets and risk-free borrowing or lending is investigated. A minimax model is established and the mathematic characteristics of the model are studied. The efficient portfolio, the efficient frontier of the analytic expression and the geometric characteristics are given, and a specific example is presented.%研究了不允许卖空条件下无风险资产借贷的最优证券投资组合问题，介绍了极大极小模型，研究了该模型的数学特征，给出了有效投资组合与有效前沿的解析表达式及其表达式的几何特征，并给出了具体算例。
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