The null space of S is the set of possible flux vectors that satisfy this steady-state condition.
From the nature.com
Geodesics are said to be time-like, null, or space-like if the tangent vector to one point of the geodesic is of this nature.
From the en.wikipedia.org
A measure space is complete if every subset of every null set is measurable.
From the en.wikipedia.org
The answer to this question lies in the fact that while representing sparse array as normal array, a lot of space is allocated for zero or null elements.