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2015, Raymond Mortini, Rudolf Rupp, “On a family of pseudohyperbolic disks”, in Elem. Math.[1], volume 70, pages 153–160:
In this elementary classroom note, we are interested in the collection of the pseudohyperbolic disks (with fixed radius and variable hyperbolic centers ) and determine explicitely[sic] with function theoretic tools the enveloppe[sic] of these disks..
