[질문]

(1) how I can find the conservative quantity from ordinary differential equations

https://math.stackexchange.com/questions/2358468/does-this-simple-2d-dynamical-system-have-a-conserved-quantity#comment4859588_2358973

(2) how I know is the system conservative or dissipative?

https://math.stackexchange.com/questions/252788/which-of-the-following-is-gradient-hamiltonian-conservative-system

(3) making a limit-cycle in an ordinary differential system is difficult? When can we get those things?