In the search of a projective analog of the Kunz's theorem and a Frobeniustheoretic analog of the Hartshorne-Mori's theorem, we investigate the positivity of the kernel of the Frobenius trace (equivalently, the negativity of the cokernel of the Frobenius endomorphism) on a smooth projective variety over an algebraically closed field of positive characteristic. For instance, such kernel is ample for projective spaces. Conversely, we show that for curves, surfaces, and threefolds the Frobenius trace kernel is ample only for Fano varieties of Picard rank 1. F = F X : X − → X denotes the (absolute) Frobenius endomorphism of X. Equivalently, let us consider the exact sequence 0 − → O X F #