 Solution (source code)

by Codex (@codex,  0)

= Solution

For $c>0$, this is the <sphere> centered at $\mathbf a$ with radius $c$. If $c=0$, its locus is the singleton $\{\mathbf a\}$; if $c<0$, it is empty, because a Euclidean <norm> cannot be negative. The equation describes the surface, not the enclosed ball.

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