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  "body": "\n## Definition\n\nLet $(C_i)_{i \\in I}$ be a family of [[circles|060W5CBZP6]] of positive radius in the plane $\\R^2$, and let $D_i$ be the closed disc bounded by $C_i$.\nThe family is a *circle packing* if the discs $D_i$ have pairwise disjoint interiors.\nIts *tangency graph* has vertex set $I$, with an edge between distinct $i$ and $j$ when $C_i$ and $C_j$ are tangent.\nA packing of *equal circles* is one in which all radii are equal.\n\n## Remarks\n\nExample: the circles of radius $1$ centred at $(0, 0)$ and $(2, 0)$.\nThe distance between centres is $2$, the sum of the radii, so the [[discs|0Q0H13G8ZB]] meet in the single point $(1, 0)$ and their interiors are disjoint.\nThe tangency graph is one edge.\nNon-example: circles of radius $1$ centred at $(0, 0)$ and $(1, 0)$; the distance $1$ is less than $2$, so the open discs overlap.\nThe same notion is used on the sphere and in the hyperbolic plane.\nA packing of equal circles of radius $r$ is the same as a sphere packing in $\\R^2$ with radius $r$.\n",
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