Stable Numerics Subroutine Library
Programming Reference Manual
Version 1.2 DD-00006-012

3.1.5 Normalized Interval

An interval X ∈IR is considered to be normalized if either   -
x≤x or a singleton   -
x=x =∅ otherwise the interval is considered to be denormalized in the first case, in the second case if one of but not both of x or -
x are ∅ the interval is considered improper. Singleton intervals are always considered normalized.