  Let T_0=(A_0,..,A_d) be a d-simplex, G_0 its centroid, S its circumsphere, O
the center of S. Let (B_i) be the points where S intersects the lines (G_0A_i),
T_1 the d-simplex (B_1,..,B_d), and G_1 its centroid. By iterating this
construction, a dynamical system of d-simplices (T_i) with centroids (G_i) is
constructed. For d=2 or 3, we prove that the sequence (OG_i) is decreasing and
tends to 0. We consider the sequences (T_{2i})_i and (T_{2i+1})_i ; for d=2
they converge to two equilateral triangles with at least quadratic speed ; for
d=3 they converge to two isosceles tetrahedra with at least geometric speed. In
this last case, we give an explicit expression of the lengths of the edges of
the limit form. We show also that if T_0 is a planar cyclic quadrilateral then
(T_n) converges to a rectangle with at least geometric speed or eventually to a
square with a speed that is conjectured as cubic. The proofs are largely
algebraic and use Grobner basis computations.
