Examples counterexamples

• Examples of 3×3 Matrices With Common Properties
  Browse verified 3×3 matrix examples for symmetry, rank, invertibility, orthogonality, idempotence, nilpotence, and noncommutativity.
• A Simple Example of a Lindelöf Space
  The real line is a simple Lindelöf space: its countable rational-interval basis lets you extract a countable subcover from any open cover.