Exercise · ZW-ORDER-OF-AN-ELEMENT-006 · ★★☆ Intermediate
The matrix \(A=\begin{pmatrix}0&-1\\1&0\end{pmatrix}\), a \(90^\circ\) rotation, belongs to the group \(GL_2(\mathbb{R})\). What is its order?
Choose one option.
Show answer
- B. 4
Explanation
\(A^2=-I\), \(A^3=-A\) and \(A^4=I\); no smaller power is the identity. Four \(90^\circ\) rotations make a full turn.
Review the concept: Order of an element
The order of an element \(a\) of a group with identity \(e\) is the least integer \(n>0\) with \(a^n=e\); if there is none, \(a\) is said to have infinite order.
\[|a|=\min\{\,n\ge 1 : a^n=e\,\}\]
More practice problems: order of an element
- ZW-ORDER-OF-AN-ELEMENT-001 What is the order of the element 3 in the group (ℤ_12,+)?
- ZW-ORDER-OF-AN-ELEMENT-002 What is the order of 2 in the group (ℤ_7^*,·) of units modulo 7?
- ZW-ORDER-OF-AN-ELEMENT-003 What is the order of the permutation (1\ 2\ 3)(4\ 5) in S_5?
- ZW-ORDER-OF-AN-ELEMENT-004 What is the order of 1 in the group (ℤ,+)?
- ZW-ORDER-OF-AN-ELEMENT-005 What is the order of 5 in (ℤ_15,+)?
- ZW-ORDER-OF-AN-ELEMENT-007 An element a of a group has order 6. What is the order of a^2?
- ZW-ORDER-OF-AN-ELEMENT-008 How many elements of order 2 does the symmetric group S_3 have?
- ZW-ORDER-OF-AN-ELEMENT-009 An element a has order 12. Which of the following powers is certain to equal the identity e?