Integer multiplication as one of the basic arithmetic functions has been
in the focus of several complexity theoretical investigations.
Ordered binary decision diagrams (OBDDs) are one of the most common
dynamic data structures for boolean functions.
Among the many areas of application are verification, model checking,
computer-aided design, relational algebra, and symbolic graph algorithms.
In this paper it is shown that the OBDD complexity of the most significant
bit of integer multiplication is exponential answering an open question
posed by Wegener (2000).