Group Theory | Few Theorems on Normalizer Centralizer & Normal Subgroups

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In this video, we discuss the following contents:
Theorem: If H,K are normal subgroups of a group G. Then HK is normal subgroup of group G.
Theorem: If H is subgroup of G, then H is subgroup of normalizer of H.
Theorem: If H is normal subgroup of G, then centalizer of H is normal subgroup of G.