We shall note here *T*=*T*_{ABC}.

**1. Let us first demonstrate that the image through T=T_{ABC} of plane E is included in the interior of a triangle**

Le’s study the restriction of *T* to the circumscribed circle of triangle ABC.

$\Gamma$ stands for the circumscribed circle to triangle ABC.

We will use the usual notations in ABC : AB = *c*, BC = *a*, CA = *b* etc..

M being a some point of plane E, we consider the barycenter M’ of points (A,MA), (B,MB) et (C, MC), so M’ = *T*(M).

First we notice that if M=A, then M=A’ =*T*_{BC} (A) where A’ is the barycenter of (B, AB) and (C, AC) (since MA = 0) and that, if M=B then M’=B’=*T*_{CA}(B) where B’ is the barycenter of (C, BC) et (A, BA).

We will demonstrate that, if M belongs to the circle arc of $\Gamma$ with summits A and B which does’nt contain C, with M$\ne$A and M$\ne$B then M’ belongs to segment ]A’B’[.

A’ = bar *(B, c); (C; b)*. So, for each point N, $c\overrightarrow {NB} + n\overrightarrow {NC} = \left( b + c \right)\overrightarrow {NA’} $ .

In the same way :

$a\overrightarrow {NC} + c\overrightarrow {NA} = \left( c + a \right)\overrightarrow {NB’} $ **(1)**.

and $b\overrightarrow {NA} + a\overrightarrow {NB} = \left( b + a \right)\overrightarrow {NC’} $ **(2)**.

Replacing N with A’ in**(1)**,we get :$c\overrightarrow {AB} + b\overrightarrow {AC} = \left( {b + c} \right)\overrightarrow {AA’} $.

Replacing N with A in **(2)** we get: $\overrightarrow {AC} = \frac{c + a}{a}\overrightarrow {AB’} $.

Replacing N with A in **(3)** we get : $\overrightarrow {AB} = \frac{b + c}{a}\overrightarrow {AC’} $.

Finally: $\frac{c\left( b + a \right)}{a}\overrightarrow {AC’} + \frac{b\left( c + a \right)}{a}\overrightarrow {AB’} = \left( b + c \right)\overrightarrow {AA’} $.

Or : $ - a\left( b + c \right)\overrightarrow {AA’} + b\left( c + a \right)\overrightarrow {AB’} + c\left( b + a \right)\overrightarrow {AC’} = \overrightarrow 0 $.

So $A = bar\left\{ {\left( {A’; - a\left( b + c \right)} \right);\left( {B’;b\left( c + a \right)} \right);\left( {C’;c\left( b + a \right)} \right)} \right\}$ (Sum of coefficients 2*bc*).

Or $A=bar\left\{ \left( A’;-a^2\left( b+c \right) \right);\left( B’;ab\left( c+a \right) \right);\left( C’;ac\left( b+a \right) \right) \right\}$(sum of coefficients 2*abc*).

In the same way :

$B=bar\left\{ \left( B’;-b^2\left( c+a \right) \right);\left( C’;bc\left( a+b \right) \right);\left( A’;ba\left( c+b \right) \right) \right\}$(Sum of coefficients 2*abc*).

et $C=bar\left\{ \left( C’;-c^2\left( a+b \right);\left( A’;ca\left( b+c \right) \right) \right);\left( B’;cb\left( a+c \right) \right) \right\}$(Sum of coefficients 2*abc*).

If M’ = *T*(M), according to homogeneity property of barycenters , M’ is the barycenter of (A;2*abc*MA), (B,2*abc*MB) and (C,2*abc*MC) then,using the property of partial barycenters, we get :

M’ = bar {(A’, $a\left( {b + c} \right)\left( { - a{\text{MA}} + b{\text{MB}} + c{\text{MC}}} \right)$ );(B’,$b\left( {c + a} \right)\left( - b{\text{MB}} + c{\text{MC}} + a{\text{MA}} \right)$);(C’,$c\left( {a + b} \right)\left( { - c{\text{MC}} + a{\text{MA}} + b{\text{MB}}} \right)$)}.

The Ptomemy’s theorem (proposition 1.2) shows that the three coeffcients are positive or null,which proves that M’ is inside triangle A’B’C ’ is on one of the edge segments of the triangle if, and only if, one of the three coefficients is null, which, still from Ptolemy’s theorem, is equivalent to say that M lies on one of the three circle arcs.

Fo instance, point M will belong to segment [B’C’]if, and only if : $ - a{MA} + b{MB} + c{MC} = 0 \Leftrightarrow {AB} \times {MC} = {BC} \times {MA} + {AB} \times {MC}$ which, from Ptolemy’s theorem, is equivalent to say that M belongs to the circumscribed circle of ABC ant that the quadrangle ABMC is convex, which is equivalent to say that M lies on the circle arc of $\Gamma$ with summits B and C that does’nt contain A. The figure on the right illustrates this property : |

**Proposition 4.1 :**The image through T_{ABC} of a some point M of plane E is a point M’ inside triangle A’B’C ’, M’ being on the edges of A’B’C ’ if, and only if, point M is on the circumscribed circle of ABC.

Let us notice that A’ = *T*_{BC}(A) may be constructed in a geometrical way as we already explained, as well as B’=TCA (B). Their construction is omitted on the following figure for more legibility.

Let us construct P = T_{AB}(M) with the method exposed before with two points.According to the partiel barycenter theorem, M’ is also barycenter of (P; MA+MB) and (C; MC).So M’ belongs to line (CP). Therefore it is the intersection point between line (CP) with line (A’B’). |

**Bijectivity of T restricted to the arc of summits A et B which does’nt contain C.**

The study made for the two points case proves that in two distincts points M of the arc with summits A and B not containing C correspond two distinct points P=T_{AB}(M) and therefore two distinct points M’ distincts on segment [A’B’]. So *T* is injective on this circle arc.

M’ is a point of segment [A’B’], distinct from A’ and B’. Let us call P the intersection point of line (CM’) with segment [AB]. Let us construct P’, symmetric point of P through I, midpoint of [AB]. Let us call H the intersection point of the perpendicular bisector of [AB] with the circle arc containing C. Then we construct point M, intersection point of line (HP’) with segment [AB].

Then we know that P=*T*_{AB}(M) and obviously M’=*T*_{ABC}(M).

**Conclusion :**

**Théorème 4.2 :** ABC being a triangle inscribed in a circle $\Gamma$, the transformation *T*_{ABC} sets a bijection from $\Gamma$ to the triangle A’B’C’ where A’=*T*_{BC} (A), B’=*T*_{CA}(B) and C ’=*T*_{AB}(C), T_{ABC} sending respectively the arcs BC, CA et AB not containing A, B and C on segments [B’C’], [C’A’] et [A’B’].

**2. Construction of the image of a some M through T.**

We only have to create the respective images P, Q, R of M through T_{BC}, T_{CA}, T_{AB} via the method exposed for two points.M’ = T (M) is therefore at the intersection of lines (AP), (BQ) and (CR) as shown on the figure beside ( we need only two of these lines of course) |

**3. Let us now search the points which have the same image as a given point M.**

**Lemma 4.3 :** A, B et C are three points non aligned of the plane, $\Gamma$ is their circumscribed circle and *R* it’s radius.

We write *T* = *T*_{ABC}.

Let us consider M$\in$E. If M is not on $\Gamma$ and if M$\ne$ O, there is eaxctly a point N verifying *T*(M) = *T*(N) : it is the image *i*(M) through inversion *i* of center O and ratio *R*^{2} ,*R* being the radius of $\Gamma$. On $\Gamma$, application T is injective.

First we notice *T*(O) = G where G is the gravity center of triangle ABC and O is the unique point whose image is G.

We suppose that M $\in \Gamma$ is fixed and that M $\ne$ O. In proposition 2.1 we saw that *T*(M)=*T*[*i* (M)].

If we put N=*i*(M), then we get *T*(N)=*T*(M).

To demonstrate that N is the unique other point whose image through *T* is the same as image of M, we only have to get that que, given a point M, there are at most two points N such as *T*(N)=*T*(M).

Let us suppose that *T*(M)=*T*(N). (A, B, C) being an affine frame of the plane, it implies that the barycentric coordinates of M and N sur (A, B, C) are equal so :

$\frac{MA}{MA + MB + MC} = \frac{NA}{NA + NB + NC}$, $\frac{MB}{MA + MB + MC} = \frac{NB}{NA + NB + NC}$ et $\frac{MC}{MA + MB + MC} = \frac{NC}{NA + NB + NC}$.

Using the inverse numbers and solving, we get : $\frac{MA}{MB} = \frac{NA}{NB}$, $\frac{MA}{MC} = \frac{NA}{NC}$ and $\frac{MB}{MC} = \frac{NB}{NC}$.

So N is at the intersection of circles $C_{A,B,\frac{MA}{MB}}$ and $C_{A,B,\frac{NA}{NB}}$ (and also $C_{B,C,\frac{MB}{MC}}$) ), which proves indeed that there are at most two points N solutions (et therefore exactly two when M $\notin \Gamma$ and M $\ne$ O since then we have *i*(M) $\ne$ M).

**4. Let us show that T is surjective on the interior of triangle A’B’C ’.**

**Lemma 4.4 :** $\Gamma$ being a circle of center O, radius *R* going through points A and B. If *k* > 0, the circles C_{A, B, k} are circles orthogonal to $\Gamma$. In particular, they are invariant through the inversion *i* of center O and ratio *R*^{2}.

**Demonstration :** We suppose that *k* > 0, *k* $\ne$ 1 and that C is the circle C_{A, B, k}. We consider a point M, intersection point of C and $\Gamma$. Naming I et J the feet of the bisectors from M in AMB, we know that [IJ] is a diameter of C and, according to proposition 1.5, the tangent to C in M is going through the midpoint of segment [IJ]. The circles C et $\Gamma$ are therefore orthogonal and proposition 1.6 proves that the circle is invariant through the inversion *i* of center O and ratio OA^{2}.

**Note 4.5 :** The lemma 4.4 applies to the case *k* = 1 considering that the perpendicular bisector of [AB] is a circle whose center is to the infinty. This perpendicular bisector is invariant through *i*.

Circle C being invariant through *i*, it is natural to get interested in it’s image through *T*

**Proposition 4.6 :** $\Gamma$being the circumscribed circle of a triangle ABC, and *k* a real number strictly positive, let us name M_{1} and M_{2} the two intersection points of C_{A,B,k} with the circle $\Gamma$ and M’_{1} et M’_{2} their respective images through *T* = *T*_{ABC}. The image through *T* of C_{A,B,k} is the segment [M’_{1}M’_{2}].

**Particular case :** If *k* = 1, C is the perpendicular bisector of [AB] and if M lies on C, M’=*T*(M) is on line (CP), P being the midpoint of [AB].

**Demonstration :** Le us name $\Gamma_1$ one of the two circle arcs of C = C_{A,B,k} of summits M_{1} and M_{2} and let us assume that M is a point of $\Gamma_1$. By definition of circle C , the foot of the bisector of M_{1} in AM_{1}B is the same as the foot of the bisector of M in AMB. Assuming that P is the symmetric point of Ithrough the midpoint of [AB]. We know the that M’=*T*(M) is on line (CP).

We set M’_{1}=*T*(M_{1}) and M’_{2}=*T*(M_{2}).

M_{1} et M_{2} beeing on $\Gamma$, we know that M’_{1} and M’_{2} are on the sides of triangle A’B’C’. Moreover we know that M’ is inside this triangle, so M’ is on segment [M’_{1} M’_{2}].

Remain to be proved that the image through T of $\Gamma_1$ is the entire segment [M’_{1} M’_{2}].

Fo this we will use a bit of topology.

The function *f* : **C** ——> **C** associated to *T* defined by :

$f(z)=\frac{\left| z-a \right|a+\left| z-b \right|b+\left| z-c \right|c}{\left| z-a \right|+\left| z-b \right|+\left| z-c \right|}$ being continuous, the image through *f* of a connex set is a connex set.

The circle arc $\Gamma_1$ being connex and included in segment [M’_{1}M’_{2}] can only be the segment [M’_{1}M’_{2}].

Moreover this proposition proves that the image through *T* of the entire plane is the inside of triangle A’B’C’.

Indeed let us assume that point M ’is a given point inside triangle A’B’C’. We consider P, intersection point of (CM’) with segment [AB] and I the symmetric point of P through the midpoint of segment [AB]. What precedes proves that, setting $k = \frac{IA}{IB} = \frac{PB}{PA}$, M’ is on the image through *T* of C_{A,B,k}.

Case where k $\ne$1 |
Cas où k = 1 |

**Thorem 4.7 :** If ABC is a triangle, the image through *T*=*T*_{ABC} of the entire plane is the inside of triangle A’B’C’, where A’, B’ et C’ are the repective images of A, B et C through *T*.

Moreover we have a method to construct the antecdent points of a point M’ through *T*.

We only have to do the preceeding construction with two circles C as on the left figure above where M_{2} and M_{3} are the antecedent points of M’ through *T*. Moreover we know that line(M_{2}M_{3}) is going through O,center of the circumscribed circle $\Gamma$to ABC, since they are images odf each other through the inversion of center O letting A, Band C invariant. Of course, if we draw the third circle C relative to points C et A, it goes through M_{2} and M_{3} as well.

If point M’ lies on the side of triangle A’B’C’, the three circles are tangent in a point located on the circumscribed circle $\Gamma$(since M’ has got an unique antecedent point through *T*) as you can see on the rght figure below.

**5. A simpler construction of T_{ABC}(M) when M lies on the circumscribed circle of ABC.**

Let us assume that M lies on the cricle arc ofe $\Gamma$ with summits B and C which does’nt contain point A. The line (A’M) re-intersects the circle $\Gamma$ in a point N. We are going to demonstrate that M’ = T(M) is on line (AN).We will name R the intersection point of line (AN) with segment [BC]. M’ = bar {(A, MA); (B, MB); (C, MC)}. According to the inscribed angle theorem we get that triangles A’MB and A’CN are similar. So we get $\frac{MC}{CA’} = \frac{NB}{BA’}$. In the same way, A’MC et A’NB are imilar. So we get $\frac{MC}{CA’} = \frac{NB}{BA’}$. Therefore M’ = bar {(A, MA); (B,$\frac{CN \times BA’}{NA’}$); (C,$\frac{NB \times CA’}{BA’}$)}. We know that R = bar {(B, RC); (C, RB)}, using the partial barycenter theorem, we now have to prove that $\frac{CN \times BA’}{NA’ \times RC} = \frac{NB \times CA’}{BA’ \times RB} \Leftrightarrow \frac{CN \times RB}{BN \times RC} = \frac{CA’}{BA’}$ (1). |

Knowing that A’ = *T*(A) = bar {(B, BA); (C, CA)}, we get $\frac{CA’}{BA’} = \frac{AB}{AC}$ so (1) $ \Leftrightarrow \frac{CN \times RB}{BN \times RC} = \frac{AB}{AC}$ **(2)**.

Triangles RBN et RAC being similar, we get : $\frac{RB}{BN} = \frac{RA}{AC}$

Triangles RBN et RAC being similar, we have : $\frac{RB}{BN} = \frac{RA}{AC}$

Triangles RAB et RCN being similar, we get : $\frac{CN}{RC} = \frac{AB}{RA}$.

We deduct from these two equalities that **(2)** est true, so M ’$\in$ (AR)=(AN).

Continuation of the demonstration

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