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## Description of constant
-$C_{36}=\Delta_4$ is the **(optimal) sphere packing density** in $\mathbb{R}^4$, i.e. the largest fraction of $\mathbb{R}^4$ that can be covered by congruent balls with disjoint interiors.
+$C\_{36}=\Delta\_4$ is the **(optimal) sphere packing density** in $\mathbb{R}^4$, i.e. the largest fraction of $\mathbb{R}^4$ that can be covered by congruent balls with disjoint interiors.
[CE2003-pack-problem] [CE2003-def-density] [CE2003-greatest-density]
More precisely, for a packing $\mathcal{P}$ in $\mathbb{R}^4$, let $P$ denote the union of all balls in the packing, and let $B(p,R)$ denote a (Euclidean) ball of radius $R$ centered at $p$. The (upper) density of $\mathcal{P}$ is