Counting excursions: symmetries, knock-ins and non-linear formula for Itô--McKean diffusions

Excursion theory is revisited on the ground of Itô–McKean diffusions. There are raised questions about symmetries, knock-in processes, excursion local time and the non-linear version of the master formula of excursions. The questions are answered due to introducing the counting excursion technique. The technique is a synthesis of straddling at time approach, the classical, potential in spirit approach, and the theory of convolution algebra of locally integrable functions, generalized later in this work for the convolutions of σ–finite measures. Some examples are presented, including the famous problem of expressing the density of first hitting time of Ornstein-Uhlenbeck process in terms of elementary functions.