An n x n matrix A is a defective matrix if it has a defective eigenvalue. Obviously, a Jordan block of dimension greater than 1, and a matrix whose Jordan canonical form has a Jordan block of dimension greater than 1, are defective matrices.

If I am not sure whether what I want to do needs planning permission, how can I find out? You can write to your Local Planning Authority and ask them to give you a ruling (or, as the Act [the Town and Country Planning Act 1947] calls it, a 'determination') whether you need planning permission or not, and if you disagree with their ruling you can appeal to the Minister.

We must accept the imperfectability of the world.

Every category is dual to its own dual, so if a statement holds in all categories so does its dual.