1. Triquetras and Porisms
2. Bogomolny, A. , Orthocenter and three equal circles, Cut-the-knot (2015), available at: http://www.cut-the-knot.com/Curriculum_Geometry/EqualCirclesOrthocenter.shtml
3. A Circle Theorem
4. Bogomolny, A. , 3 circles having the same radius, Cut-the-knot (2015), available at: http://www.cut-the-knot.org/proofs/3circlesFormal.shtml
5. Wikipedia, Johnson circles (2014) available at http://en.wikipedia.org/wiki/Johnson_circles