Abstract
We study the quadratic Lagrange spectrum defined by Parkkonen and Paulin by considering the approximation by elements of the orbit of a given real quadratic irrational number for the action by homographies and anti-homographies of \(PSL_2(\mathbf{Z})\) on \(\mathbf{R}\cup \{\infty \}\). Our approach is based on the theory of continued fractions.
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Acknowledgments
I am very pleased to thank Jouni Parkkonen and Frédéric Paulin for having encouraged me to write this note and for fruitful correspondence. Many thanks to the referee for a very careful reading.
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Bugeaud, Y. On the quadratic Lagrange spectrum. Math. Z. 276, 985–999 (2014). https://doi.org/10.1007/s00209-013-1230-1
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DOI: https://doi.org/10.1007/s00209-013-1230-1