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Relations/topology/Simons: Let R be a binary relation and a is a class of objects - Definition left closed under R: is a, if no R-relationship runs from the outside to the inside - E.g. If nobody third owes Jack and Jill money, the pair is left closed under debt - while everyone can owe something the other or any third party (external). - conversely to that: Definiton right closed under R: if no R-relationship runs from the inside to the outside - E.g. neither Jack nor Jill owes a third something, regardless of whether they are creditors of a third party or among themselves.
Parts Oxford New York 1987