The Young bouquet and its boundary

The classification results for the extreme characters of two basic “big” groups, the infinite symmetric group $S(\infty)$ and the infinite-dimensional unitary group $U(\infty)$, are remarkably similar. It does not seem to be possible to explain this phenomenon using a suitable extension of the Schur–Weyl duality to infinite dimension. We suggest an explanation of a different nature that does not have analogs in the classical representation theory.
We start from the combinatorial/probabilistic approach to characters of “big” groups initiated by Vershik and Kerov. In this approach, the space of extreme characters is viewed as a boundary of a certain infinite graph. In the cases of $S(\infty)$ and $U(\infty)$, those are the Young graph and the Gelfand–Tsetlin graph, respectively. We introduce a new related object that we call the Young bouquet. It is a poset with continuous grading whose boundary we define and compute. We show that this boundary is a cone over the boundary of the Young graph, and at the same time it is also a degeneration of the boundary of the Gelfand–Tsetlin graph.
The Young bouquet has an application to constructing infinite-dimensional Markov processes with determinantal correlation functions.