Let G be a finite group of order n, where n is prime.
If possible, let H be a subgroup of order m,say.
Then .
But by Lagrange's theorem m is a divisor of n.
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Lagrange's Theorem:
The order of a subgroup of a finite group divides the order of the group.
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Also since n is prime, either m=1 (or) m=n.
(or) H=G.
But these two are improper subgroups of G.
Any group of prime order does not have any proper subgroup.
Thus the total number of subgroups of a group of prime order is 2.
They are and G itself.
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