Unfolding all definitions, a group $(G,\ast)$ fulfills the following properties:
Axiom: Axioms of Group
 closure: $x \ast y\in G$ for all $x,y\in G$.
 associativity: \((x\ast y)\ast z=x\ast (y\ast z)\,\).
 existence of neutral element: There is an element $e\in G$ with $e\ast x=x\ast e=x$ for all $x\in G$.
 existence of inverse: For all $x\in G$ there exists an $x^{1}\in G$ with $x\ast x^{1} =x^{1}\ast x=e$.
Notes
 For technical reasons, these axioms are not minimal.
 It is also possible to define a group if we require only the existence of a leftneutral (respectively a rightneutral), and the existence of leftinverse (respectively a rightinverse) elements.
 The reader might encounter this approach in some sources.
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References
Bibliography
 Knauer Ulrich: "Diskrete Strukturen  kurz gefasst", Spektrum Akademischer Verlag, 2001
 Lang, Serge: "Algebra  Graduate Texts in Mathematics", Springer, 2002, 3rd Edition