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A156235 Number of irreflexive binary relations on the power set P(N) of an n-element set N as restricted below. 0
1, 1, 4, 198, 209342 (list; graph; refs; listen; history; text; internal format)



Each enumerated irreflexive relation R has these restricting properties:

Let (A,B) and (C,D) be arbitrary elements of R. Then

i) A and B are nonempty subsets of N,

ii) A and B are disjoint, and

iii) if (A,B) is not equal to (C,D) and A intersect C is nonempty, then B and D are disjoint.

Each a(n) includes the empty relation. Each relation R may contain any number of elements from 0 to n^2-n.

Inspired by considering less-restricted gift-exchange scenarios than in A053763.

Essentially, the scenarios here relax (somewhat but not entirely) noted restrictions iii) and iv) given there to allow joint giving and joint receiving.

More generally, these relations could be considered distribution networks (or even possible economies, in some sense) for goods and/or services whenever an entity cannot directly distribute to itself or to another entity of which it is a part and whenever an entity cannot (jointly) distribute directly to a second entity in more than one way (e.g., as part of two larger entities).


Table of n, a(n) for n=0..4.


One of the 209342 irreflexive relations corresponding to a(4) is

R = {({1},{2}), ({2},{1}), ({3,4},{1,2}), ({1,4},{3}), ({2},{3,4})}.

Notice how the last three ordered pairs correspond to jointly giving and/or receiving gifts.


Cf. A000392, A002378, A028243, A053763.

Sequence in context: A263422 A065246 A297061 * A034862 A174776 A216932

Adjacent sequences:  A156232 A156233 A156234 * A156236 A156237 A156238




Rick L. Shepherd, Feb 06 2009



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Last modified June 13 07:45 EDT 2021. Contains 344981 sequences. (Running on oeis4.)