Algebraic Limit Cycles

In the qualitative theory of differential equations in the plane [Formula: see text], one of the most difficult objects to study is the existence of limit cycles. Here, we summarize some results and open problems on the algebraic limit cycles of the planar polynomial differential systems. More precisely, (i) we study the algebraic limit cycles of the polynomial differential systems of degree 2; (ii) we provide the maximum number of generic algebraic limit cycles that the polynomial differential systems of degree n can exhibit; (iii) we show using the algebraic limit cycles that any finite configuration of limit cycles can be realized by some polynomial differential system; and (iv) we provide the maximum number of algebraic limit cycles formed by circles that a polynomial differential system of degree n can exhibit.