Is there a generalized Feuerbach point for an irregular non-Euclidean triangle?

2011-04-07T16:41:30Z

Is the circle externally tangent to the three excircles of an irregular non-Euclidean triangle internally tangent to the incircle of the triangle, the tangent point being a generalized Feuerbach point? In Euclidean plane geometry, the circle externally tangent to the excircles of an irregular triangle is internally tangent to the incircle at the Feuerbach point. My calculations indicate that this should similarly obtain for a non-Euclidean triangle, but I have found no proof. (It is possible that my calculations could, with difficulty, be turned into an analytic proof.) A circle is internally (externally) tangent to another circle if the distance between their centers is the difference (sum) of their radii. A triangle is irregular if it is not equilateral.

In response to a helpful comment, my calculations indicate that the additional tangency does not obtain in higher dimensions, so I have revised the question to apply to only a two-dimensional space.

Answer by Robert A. Russell for Is there a generalized Feuerbach point for an irregular non-Euclidean triangle?

2012-02-01T02:40:42Z

Akopyan pointed out to me that Hart proved this. Hart's 1861 article can be found at http://books.google.com/books?id=y9xEAAAAcAAJ&pg=PA260#v=onepage&q&f=false. Akopyan's article in translation appears at http://arxiv.org/pdf/1105.2153.pdf. My dynamic illustration appears at http://demonstrations.wolfram.com/NonEuclideanTriangleContinuum.

Is there a generalized Feuerbach point for an irregular non-Euclidean triangle? - MathOverflow

Is there a generalized Feuerbach point for an irregular non-Euclidean triangle?

Answer by Robert A. Russell for Is there a generalized Feuerbach point for an irregular non-Euclidean triangle?