Define a complete lattice and give one example.
Ans: A lattice (L, ≤) is said to be a complete lattice if, and only if every non-empty subset S of L has a greatest lower bound and a least upper bound. Let A be set of all real numbers in [1, 5] and ≤ is relation of 'less than equal to'. Then, lattice (A, ≤) is a complete lattice.