子模函数(次模函数) submodular function
https://www.cs.princeton.edu/courses/archive/fall16/cos521/Lectures/lec16.pdf
http://karthik.ise.illinois.edu/courses/ie511/lectures-sp-19/13.pdf
子模函数定义
https://www.cs.princeton.edu/courses/archive/fall16/cos521/Lectures/lec16.pdf
http://karthik.ise.illinois.edu/courses/ie511/lectures-sp-19/13.pdf
子模函数定义