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Let $S= \{0, 1, 2, 3, 4, 5, 6, 7\}$ and $⊗$ denote multiplication modulo $8,$ that is, $x ⊗ y= (xy) \mod 8$

Prove that $( \{ 0, 1\}, ⊗)$ is not a group.

Write three distinct groups $(G, ⊗)$ where $G ⊂ S$ and $G$ has $2$ elements.
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A $1$ is the identity element. Inverse does not exist for zero. So, it is not a group.

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Then how can A be a group?
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Sorry I can't get your question.Actual question is to prove A is not a group.I also proved it.Is there is any mistake?
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Sorry. i misread the question and didn't see the A, B separation in answer.
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it's ok :)