I want to understand the definition of the spectrum and therefore I want to know, what it means that a model has n elements or that a model is of size n. What is said to be an element? Are these only the elements of the universe?
Furthermore, I am trying to find a sentence $\phi$ without any "="-relation, so that $sp(\phi) = \{n \in \mathbb{N}: n > 5\}$. Any idea on that?