Diameter of a square In "Differential Geometry" by Heinrich W. Guggenheimer, I found the terminology "diameter of a square" when referring to the Peano curve. Clearly from context this refers to the diagonal of the square. Is this a typo or is this terminology known? A Google search turns up nothing useful.


The diameter of $S⊆M$ for $(M,d)$ a metric space is
$$\operatorname{diam}(S) := \sup_{x,y \in S} d(x,y)$$
For a square in $\mathbb{R}^2$, under the Euclidean metric, this would correspond precisely to the diagonal of the square.
