A set of points which do not lie on any of a certain class of hyperplanes.
Drinfeld's Symmetric Space
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References
Teitelbaum, J. "The Geometry ofReferenced on Wolfram|Alpha
Drinfeld's Symmetric SpaceCite this as:
Weisstein, Eric W. "Drinfeld's Symmetric Space." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DrinfeldsSymmetricSpace.html