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A fundamental concept in the theory of ordinal numbers is a transitive set, which we want now to introduce formally:

## Definition: Transitive Set

A transitive set $$Z$$ is one in which the following implication is always fulfilled:

$$x\in y\wedge y\in Z\Longrightarrow x\in Z,$$

i.e. if $x$ is element of $y$ and $y$ is element of $Z$, then $x$ is also an element of $Z.$ This is equivalent to the following: If $y\in X$, then $y\subseteq X$ (i.e. every element $y$ of $Z$ is also its subset).

Because of the axiom of foundation, by which no set can contain itself, we can even require that $y$ is a proper subset of $Z,$ and the definition of a transitive set can then be written as follows:
$$y\in Z\Longrightarrow y\subset Z.$$

## 2.Lemma: Any Set is Subset of Some Transitive Set - Its Transitive Hull

### Bibliography (further reading)

 Hoffmann, Dirk W.: “Grenzen der Mathematik – Eine Reise durch die Kerngebiete der mathematischen Logik”, Spektrum Akademischer Verlag, 2011