A 
-system of a set 
is a sequence of subsets 
, 
, ..., 
of 
,
among which some may be empty or coinciding with each other.
p-System
See also
Inclusion-Exclusion Principle, k-Subset, SubsetExplore with Wolfram|Alpha
References
Comtet, L. Advanced Combinatorics: The Art of Finite and Infinite Expansions, rev. enl. ed. Dordrecht, Netherlands: Reidel, pp. 176-177, 1974.Referenced on Wolfram|Alpha
p-SystemCite this as:
Weisstein, Eric W. "p-System." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/p-System.html