1, section definition: the section shape perpendicular to the beam axis. Not all smooth flows have cross-sections, and an obvious necessary condition is that flows cannot have singularities. The cross section was introduced by Poincare ((J.-) H). Through the cross section, the relationship between smooth flow and discrete dynamic system generated by differential homeomorphism can be established

2. Mathematics is a universal means for human beings to strictly describe abstract structures and patterns of things, and can be applied to any problems in the real world. All mathematical objects are artificially defined in essence. In this sense, mathematics belongs to formal science, not natural science. Different mathematicians and philosophers have a series of views on the exact scope and definition of mathematics.