  We study the involutions belonging to the class of 321 avoiding permutations.
We calculate the algebraic generating functions of the set containing the
involutions avoiding 321 and of some of its subsets. Precisely we determine the
algebraic generating functions of the involutions that are expansions of 12, of
those expansions of 21, of the simple ones and of their expansions. The
graphics of the simple involutions are caracterized. Being the simple
involutions avoiding 321 counted by Riordan's numbers, a combinatoric
interpretation of the results is illustrated through a class of Motzkin paths.
Another interpretation is given through Dyck paths.
