Exercise · ZW-FERMAT-PRIMALITY-TEST-007 · ★★☆ Intermediate
Use the test with base 2 for n = 9. What do you conclude?
Choose one option.
Show answer
- C. 9 is composite, because \( 2^8 \equiv 4 \pmod{9} \)
Explanation
We have \( 2^8 = 256 = 28\cdot 9 + 4 \), so the remainder is 4 rather than 1; the congruence fails and 9 is composite.
Review the concept: Fermat primality test
The Fermat primality test is a probabilistic criterion for an integer \(n>1\): it declares \(n\) composite if, for some base \(a\) coprime to \(n\), the power \(a^{n-1}\) is not congruent to 1 modulo \(n\).
\[a^{n-1} \equiv 1 \pmod{n}\]
More practice problems: fermat primality test
- ZW-FERMAT-PRIMALITY-TEST-001 What does a^n-1 not≡ 1 pmodn with gcd(a, n) = 1 tell you?
- ZW-FERMAT-PRIMALITY-TEST-002 What is 2^4 bmod 15 ?
- ZW-FERMAT-PRIMALITY-TEST-003 What is the name of the test that uses a^n-1 ≡ 1 pmodn to detect composites?
- ZW-FERMAT-PRIMALITY-TEST-004 What condition must the base a satisfy in the Fermat test?
- ZW-FERMAT-PRIMALITY-TEST-005 If n is prime and gcd(a, n) = 1, what holds?
- ZW-FERMAT-PRIMALITY-TEST-006 Compute 3^6 bmod 7 and state whether the test with base 3 concludes that 7 is prime or composite.
- ZW-FERMAT-PRIMALITY-TEST-008 For n = 13 and a = 5, what is the remainder of 5^12 bmod 13 ?
- ZW-FERMAT-PRIMALITY-TEST-009 For n = 21 and a = 2, what result does the Fermat test give?
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