Exercise · ZW-ZERO-DIVISOR-009 · ★★★ Advanced
How many zero divisors does \(\mathbb{Z}_{21}\) have?
Choose one option.
Show answer
- C. \(8\)
Explanation
They are the nonzero non-units: \(21-1-\varphi(21)=20-12=8\) (the multiples of 3 or 7: 3, 6, 7, 9, 12, 14, 15, 18).
Review the concept: Zero divisor
A zero divisor of a ring \(R\) is an element \(a\neq0\) for which there is another element \(b\neq0\) of \(R\) whose product with \(a\) is zero.
\[ab=0,\quad a\neq0,\ b\neq0\]
More practice problems: zero divisor
- ZW-ZERO-DIVISOR-001 In ℤ_12, which of these elements is a zero divisor?
- ZW-ZERO-DIVISOR-002 How many zero divisors does ℤ_10 have?
- ZW-ZERO-DIVISOR-003 Which of these rings has no zero divisors?
- ZW-ZERO-DIVISOR-004 In ℤ_15, which b≠0 satisfies 6b=0?
- ZW-ZERO-DIVISOR-005 In the ring of 2×2 real matrices, which of these matrices is a zero divisor?
- ZW-ZERO-DIVISOR-006 Which statement is true in a ring R with identity, R≠0?
- ZW-ZERO-DIVISOR-007 In ℤ×ℤ, which of these elements is a zero divisor?
- ZW-ZERO-DIVISOR-008 In ℤ_8, which pair with x≠ y satisfies 4x=4y?