1. The problem statement, all variables and given/known data Hi, i have the following system of equation. In the task is that system have periodic solution and have to be used polar coordinates. 2. Relevant equations x'=1+y-x^2-y^2 y'=1-x-x^2-y^2 3. The attempt at a solution After transfer to polar system i tried to use the method of variation of parameters, but without success.