Dear Darij

in Hyacinthos 7408, you wrote

> Let ABC be a triangle and AA', BB', CC' its altitudes.

> We have the following theorems:

>

> (1a) The Euler lines of triangles HB'C', HC'A', HA'B' concur

> at one point, namely the Schiffler point of triangle

> A'B'C'. It lies on the Euler line of triangle A'B'C'.

>

> (1b) The Euler lines of triangles AB'C', BC'A', CA'B' concur

> at one point on the nine-point circle of triangle ABC.

>

> Note that (1a) follows from the definition of the Schiffler

> point (H is the incenter of triangle A'B'C')

Note that if an angle of ABC is obtuse, the incenter of A'B'C' is

not H, but the corresponding vertex of ABC.

> and (1b) is a result of Victor Thebault.

Notethat the common point is the center of Jerabek hyperbola.

Friendly. Jean-Pierre