A class of area-preserving maps of the form
 |  |  |
(1)
|
 |  |  |
(2)
|
which maps circles into circles but with a twist resulting from the 
term.
Twist Map
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Cite this as:
Weisstein, Eric W. "Twist Map." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/TwistMap.html