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Lagrange's theorem

A fundamental theorem in group theory stating that the order of a subgroup divides the order of the group.
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The statement of the theorem

If HH is a subgroup of a finite group GG, then ∣G∣=[G:H]⋅∣H∣|G| = [G:H] \cdot |H|, where [G:H][G:H] is the index of HH in GG.
Source: Wikipedia