Count in relational algebra using Ω = { π, σ, ⋈, ⋉, β, x, ∪, ∩ , - } schema(author)={name, newspaper}


The table contains a list of authors. Each author works for a newspaper. So a single newspaper may employ multiple authors. How can I get the names of the newspapers that have at least two authors working for them? My algebra consists of:

Ω = { π, σ, ⋈, ⋉, β, x, ∪, ∩ , - }


There is no count in the algebra. So I am limited to the above Ω. Any ideas how I can implement counting?
Hint: to express "the set $S$ has size $\ge 1$" in propositional logic (without using the "size" operator), you can write $\exists x . x \in S$.  How would you express "the set $S$ has size $\ge 2$"?  Does that give you any ideas for how to use relational algebra to solve your question?