FEUERBACH POINT PROBLEM

Lemma 1

Let ABC be a triangle. The Oi line intersects AB at C'.The circle (Oa) with diameter AI intersects the circumcircle again at A". The line C'A" intersects (Oa) again at Ca.Ca is the orthogonal projection of A on IC.

Lemma 2

Let ABC be a triangle and Ba, Ca the orthogonal projeections of A on IB, IC, resp. The line BaCa intersects AC, AB at B'a, C'a, resp. B'a,C'a are the midpoints of AC, AB, resp.

Lemma 3

Let ABC be a triangle. Denote C' = the midpoint of AB, Ma, Mb = the midpoints of AI, BI, resp. and Ba = the orthogonal projection of A on BI. The points Ma, C', Mb, Ba are concyclic. The circle is the NPC of AIB.

Problem
By David Nguyen

Rewritten with cyclic notation
Let ABC be a triangle. The OI line intersects BC, CA, AB at A', B', C', resp.  
Denote
(Oa) = the circle with diameter AI
A" = the 2nd intersection of the circumcircle and (Oa)
Ba = the 2nd intersection of B'A" and (Oa)
Ca = the 2nd intersection of C'A" and (Oa)
B'a = BaCa ∩ AC  , C'a = BaCa ∩ AB

(Oab) = the circumcircle of OaB'aCa
(Oac)= the circumcircle of OaBaC'a
The 2nd intersection of (Oab), (Oac) is the Feuerbach point

Proof
By the lemmata, the problem is equivalent to the following problem
Let ABC be a triangle and A'B'C' the medial triangle
Denote
A", B", C" = the midpoints of IA, IB, IC, resp.
The circumcircles of A'B"C", B'C"A", C'A"B" concur at the Feuerbach point
True, since the circumcircles of A'B"C", B'C"A", C'A"B" are the NPCs of IBC ICA, IAB resp. and they concur at the Poncelet point of ABCI = the Feuerbach point.

Mail Antreas P. Hatzipolakis