2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/96036In [Phys. Rev. A 70, 062101 (2004)] Gibbons et al. defined a class of discrete Wigner functions W to represent quantum states in a finite Hilbert space dimension d. I characterize a set C_d of states having non-negative W simultaneously in all definitions of W in this class. For d<6 I show C_d is the convex hull of stabilizer states. This supports the conjecture that negativity of W is necessary for exponential speedup in pure-state quantum computation.7 pages, 2 figures, RevTeX. v2: clarified discussion on dynamics, added refs., published versionQuantum Physicstext