As the minimum feedbackvertexset problem is NP-complete, it is hard to be solved exactly.
最小反馈节点集问题是一个NP完全问题，很难准确求解。
2
A subset of the vertexset of a graph is a feedbackvertexset of the graph if the resulting graph is acyclic after removing the vertex subset from the graph.
若从一个图中去掉某些顶点后得到的导出子图是无圈图，则所去的那些顶点组成的集合就是原图的反馈点集。
3
A subset of the vertexset of a graph is a feedbackvertexset of the graph if the resulting graph is a forest after removing the vertex subset from the graph.