Proof
(related to Corollary: Justification of Subsets and Supersets)
Remark: In particular, a subset can be empty, which is ensured by the axiom of empty set.
∎
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References
Bibliography
- Hoffmann, Dirk W.: "Grenzen der Mathematik - Eine Reise durch die Kerngebiete der mathematischen Logik", Spektrum Akademischer Verlag, 2011
- Ebbinghaus, H.-D.: "Einführung in die Mengenlehre", BI Wisschenschaftsverlag, 1994, 3th Edition