in interval space, 
would hence correspond to
The map for a real number in interval space, would hence correspond to
![-
X ={r∈R :x=x =r}= [r,r]](StabNumRef_PG73x.png)
This is called a singleton interval, where 
. The reverse map on a singleton interval is defined as being 
. For
non-singleton intervals the map 
is generally defined as the midpoint between the interval limits, however
multiple further definitions exist - see the chapter on Interval Intrinsic Functions for a listing on the supported
methods.