Involutions avoiding 4321 and another pattern of length four

Source abstract

We enumerate all families of involutions avoiding a classical pattern of length four together with 4321. While the enumeration of permutations avoiding two classical patterns is by now well understood, the case of pattern-avoiding involutions presents additional structural constraints that require dedicated techniques. In particular our main enumerative tool is Biane's bijection between involutions and labelled Motzkin paths. The paper provides exact bivariate generating functions (taking into account length and number of descents), and structural descriptions for all except one family. In the remaining case we provide a functional equation for the generating function and express it as an explicit continued fraction. We make extensive use of experimental and software-supported methods.