I've been grading calculus exams this week, and even though my students did quite well overall there were a couple of errors that I see every semester. Errors that make mathematicians' heads explode.

Let Ω ⊂ ℝp, p ϵ ℕ* be a nonempty subset and B(Ω) be the Branch lattice of all bounded real functions on a Ω, equipped with sup norm. Let 𝑋 ⊂ 𝐵(Ω) be a linear sublattice of 𝐵(Ω) and 𝐴: 𝑋 → 𝑋 be a ...