It happens that I proved an elementary result about the commutant (or centralizer) of a matrix; the result is elementary but nice and can be useful; the method is also elementary. my first reaction was "it is known", but I did not find, I asked colleagues , no one knows. Then I decided to ask on Mathoverflow if the result is known; the question was refused then deleted. Taking this into account I went to stackexchange, same reaction. May I ask : what is the good forum to ask colleagues about a new (or not ?) result? I apologize for my poor English.