What does $|p\;\square\;q|$ mean?

Question

Given that $p$ and $q$ are both integers, what does $|p\;\square\;q|$ mean? It came up in Theorem 3 of the article “On the Topology of Celtic Knot Designs” from the mathematics department of California Polytechnic State University (Link below). The article explores methodology for determining how many unique strands a given Celtic knot or link will contain. http://www.mi.sanu.ac.rs/vismath/fisher/index.html#r5

Answer 1

To cut down on unanswered questions, I post this.

Your copy is broken, and should instead read $|p-q|,$ as you can see here.