Exercise · ZW-MULTIPLICATIVE-ORDER-002 · ★★☆ Intermediate
What is \( \operatorname{ord}_5(4) \)?
Choose one option.
Show answer
- D. 2
Explanation
Since \( 4^2 = 16 \equiv 1 \pmod{5} \) and no smaller exponent works, the order is 2. Answering 4, which is \( \varphi(5) \), is a common mistake.
Review the concept: Multiplicative order
The multiplicative order of \( a \) modulo \( n \), \( \operatorname{ord}_n(a) \), is the smallest positive exponent that returns \( a \) to the unit class modulo \( n \).
\[\operatorname{ord}_n(a)=\min\{k\ge 1: a^k\equiv 1 \pmod{n}\}\]
More practice problems: multiplicative order
- ZW-MULTIPLICATIVE-ORDER-001 What is the definition of the multiplicative order of a modulo n ?
- ZW-MULTIPLICATIVE-ORDER-003 What condition must a satisfy for ord_n(a) to exist?
- ZW-MULTIPLICATIVE-ORDER-004 If ord_n(a)=k and a^m ≡ 1 pmodn , what relation must hold?
- ZW-MULTIPLICATIVE-ORDER-005 What is ord_10(3) ?
- ZW-MULTIPLICATIVE-ORDER-006 Compute ord_9(2) .
- ZW-MULTIPLICATIVE-ORDER-007 If ord_13(a)=6 , what is ord_13(a^2) ?
- ZW-MULTIPLICATIVE-ORDER-008 How is ord_n(a^-1) related to ord_n(a) , when a is invertible modulo n ?
- ZW-MULTIPLICATIVE-ORDER-009 It is claimed that ord_7(a)=ord_7(a^2) for every invertible a . Which value of a disproves the clai…