Recall that a $d$-VASS is a finite automaton, where transitions are labelled with $d$-dimensional vectors over integers. A configuration of a $d$-VASS is a pair of a state and a $d$-dimensional vector over naturals. Bounded VASS (BoVASS) are a variant of the classic VASS model where all values in all configurations are upper bounded by a fixed natural number, encoded in binary in the input. It is easy to see that the reachability is in PSPACE for BoVASS and it is not hard to show NP-hardness.
In 2013 Fearnley and Jurdziński proved that the reachability problem in this model is PSPACE-hard already in dimension 1. We investigate the complexity of the reachability problem when the BoVASS model is extended with branching transitions (BoBrVASS). Branching transitions create two independent copies of the system splitting the configuration vector among them. For BoBrVASS it is easy to show that the reachability problem is in EXPTIME and PSPACE hardness follows from the results about BoVASS. Recently we proved that the reachability problem is EXPTIME-hard for $d$-BoBrVASS when $d \ge 2$, leaving the case for $d = 1$ as an open problem for Autobóz.
This problem is also suggested by Michał Pilipczuk.