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Lagrange's theorem (group theory) : ウィキペディア英語版
Lagrange's theorem (group theory)

Lagrange's theorem, in the mathematics of group theory, states that for any finite group ''G'', the order (number of elements) of every subgroup ''H'' of ''G'' divides the order of ''G''. The theorem is named after Joseph-Louis Lagrange.
== Proof of Lagrange's Theorem ==
This can be shown using the concept of left cosets of ''H'' in ''G''. The left cosets are the equivalence classes of a certain equivalence relation on ''G'' and therefore form a partition of ''G''. Specifically, ''x'' and ''y'' in ''G'' are related if and only if there exists ''h'' in ''H'' such that ''x = yh''. If we can show that all cosets of ''H'' have the same number of elements, then each coset of ''H'' has precisely |''H''| elements. We are then done since the order of ''H'' times the number of cosets is equal to the number of elements in ''G'', thereby proving that the order of ''H'' divides the order of ''G''. Now, if ''aH'' and ''bH'' are two left cosets of ''H'', we can define a map ''f'' : ''aH'' → ''bH'' by setting ''f''(''x'') = ''ba''−1''x''. This map is bijective because its inverse is given by f^(y) = ab^y\mbox
This proof also shows that the quotient of the orders |''G''| / |''H''| is equal to the index (: ''H'' ) (the number of left cosets of ''H'' in ''G''). If we allow ''G'' and ''H'' to be infinite, and write this statement as
:\left|G\right| = \left(: H\right ) \cdot \left|H\right|\mbox
then, seen as a statement about cardinal numbers, it is equivalent to the Axiom of choice.

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