Q: Verify (ℤ₆,+) is a group. Is (ℤ₆,×) a group? (ℤ₆,+): closure ✓ (mod 6), assoc ✓, identity 0 ✓, inverse -a ✓: GROUP! (ℤ₆,×): identity 1 ✓, but 2 has NO inverse (2x ≡ 1 mod 6: impossible - even!)! NOT a group (only units {1,5} form group!)!