Hiroshi Okumura and Masayuki Watanabe,  The Twin Circles of Archimedes in a Skewed Arbelos,
Forum Geometricorum 4 (2004) 229-251.

Abstract: Any area surrounded by three mutually touching circles is called a skewed arbelos. The twin circles of Archimedes in
the ordinary arbelos can be generalized to the skewed arbelos. The existence of several pairs of twin circles, under certain conditions, is demonstrated.

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