From now on (January 6, 2026), I have decided to slow down my contributions to Math SE. The reason : for now about 2 years, there exist on this site a group of anonymous downvoters having no consideration (I say it in a soft manner) for people not thinking along their lines of thought. The last example being this anonymous downvoting of one of my (modest) contributions.

I am a retired professor (Lyon, France), still with some academic research activities. Having more free time now, I devote it to sharing knowledge, especially in mathematics and its links with computer science.

Some of my answers (a few of them recent) that can be of interest:

Analysis:

PICTURE: Roots of the partial sum of exp(z)

The highest power of 2 less than x

Solving a functional equation using group theory

Infinite product for the cardinal sine

Lemniscates

A recurrent sequence involving Fibonacci sequence

Linear algebra:

Decreasing importance of determinants but...

Eigenvalues of permutation matrices

Relationship quaternions-complex numbers

A special determinant

General Algebra:

Finite fields: polynomial factorization

Non commutative ring of order 125

Geometry:

Two parameterizations for quadrilaterals

Equilateral triangle in a triangle

Sierpinski triangle

Trilinear coordinates

Playing with areas in a 5 pointed star

Geodesic triangle in a cone

Equation $SS_1-T^2=0$ of a cone tangent to a quadric

nD Simplex coordinates of vertices

Hyperbolic Geometry

Probability:

Flock of sheep

Markov chain

Geometric probability