#Lagrange's Theorem
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In the mathematical field of group theory, Lagrange's theorem is a theorem that states that for any finite group G, the order (number of elements) of every subgroup of G divides the order of G. The theorem is named after Joseph-Louis Lagrange. The following variant states that for a subgroup
H
H
of a finite group
G
G
...
Read up on the proof. If it's too complicated, I recommend reading any undergraduate textbook on group theory. Lagrange's Theorem is pretty much always proved since the proof is easy to explain.