I am reading an article about counting hexagonal p-minos.
In page 3 (of the pdf) we define a boundoing hexagon for a p-mino, In page 6 the article claims that delta - the number of cells inside and on the boundry of the bounding hexagon equal to $s1+s3+s5$ choose $2$ -...
Does anyone have any idea for why this is true ? I have no clue to even why it looks like something in the form of a choose b -...