Stable Numerics Subroutine Library
Programming Reference Manual
Version 1.1 DD-00006-011

3.1.2 Empty Interval

An empty interval X =∅  for X ∈IR  is defined as a singleton where

       -
X ={x= x= ∅}= ∅

where ∅̸∈ R  . Hence no arithmetic can be performed on an empty interval. On a computer ∅  is represented by qNaNs (Quiet-Not-a-Number) hence causing any arithmetic to yield further qNaNs.