Question:

A noncyclic group among the following is

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If every non-identity element in a group of order 4 has order 2, it is the non-cyclic Klein 4-group.
  • $(\{1, \omega, \omega^2\}, \cdot)$
  • $(Z_4, +_4)$
  • $(\{1, 3, 5, 7\}, \times_8)$
  • $(Z, +)$
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The Correct Option is C

Solution and Explanation

Step 1: Concept
A group is cyclic if there exists an element $a$ such that every element can be written as $a^n$.

Step 2: Meaning

We test the powers of elements in each set under the given operation.

Step 3: Analysis

In $(\{1, 3, 5, 7\}, \times_8)$, the square of every element is 1: $3^2=9 \equiv 1$, $5^2=25 \equiv 1$, $7^2=49 \equiv 1$. No single element can generate the whole set $\{1, 3, 5, 7\}$.

Step 4: Conclusion

Since no generator exists, this group (known as the Klein 4-group $V_4$) is non-cyclic. Final Answer: (C)
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