  This is a revised and slightly expanded version. We point out that in the
previous summary, "without cohomology" should really read "almost without
cohomology" because of the proof of Lemma 2, that the idea to consider
effective motives divisible by the Lefschetz motive was anticipated by Serre in
letters to Gilles Lachaud and Marc Perret, and finally that the birational
invariance of the number of points modulo q in fact follows from a 1983 Comptes
Rendus note of Torsten Ekedahl (I am grateful to Antoine Chambert-Loir for
making this observation and indicating this reference).
