  We are convinced of the usefulness of sketches and diagrams during
mathematical work but the observation is made in our practices that they are
not spontaneously used by students. In order to study the understanding and use
of sketches by mathematics students, we designed and then proposed a test at
different university levels. The test consists of five exercises.The first
concerns different representation registers of a set of numbers, the second on
a graphic proof of an implicative algebraic proposition and the last three on
the graphic approch to the notions of injectivity, surjectivity, bijectivity in
the context of the analysis. The sketches, proposed or requested in each
exercise, are intended to be aids to changes of register and reasoning. We
present what motivated the choices and developments of the exercises then we
analyze the results of these tests. In each case, we see difficulties in
understanding and the sketches proposed, which leads to think that the sketch
must be the subject of specific work at the university level.
