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Mao-cheng Cai Xiaotie Deng Wenan Zang A Min-Max Theorem on Feedback Vertex Sets. 73-86 1999 conf/ipco/1999 IPCO http://link.springer.de/link/service/series/0558/bibs/1610/16100073.htm db/conf/ipco/ipco1999.html#CaiDZ99