Polar varieties, Bertini's theorems and number of points of singular complete intersections over a finite field

摘要

Let V is contained in P~n(F_q) be a complete intersection defined over a finite field F_q of dimension r and singular locus of dimension at most s, and let π : V → P~(s+1)(F_q) be a generic linear mapping. We obtain an effective version of the Bertini smoothness theorem concerning π, namely an explicit upper bound of the degree of a proper Zariski closed subset of P~(s+1)(F_q)) which contains all the points defining singular fibers of π. For this purpose we make use of the concept of polar variety associated with the set of exceptional points of π. As a consequence, we obtain results of existence of smooth rational points of V, that is, conditions on q which imply that V has a smooth F_q-rational point. Finally, for s = r - 2 and s = r - 3 we estimate the number of F_q-rational points and smooth F_q-rational points of V.