2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/19160The shape of an electronic droplet in the quantum Hall effect is sensitive to gradients of the magnetic field, even if they are placed outside the droplet. Magnetic impurities cause a fingering instability of the edge of the droplet, similar to the Saffman-Taylor fingering instability of an interface between two immiscible phases. We discuss the fingering instability and some algebraic aspects of the electronic states in a strong nonuniform field.Based on the lecture given at ``Statistical Field Theory'', 18-23 June 2001, Como, Italy; 14 pages, 1 figureMesoscale and Nanoscale PhysicsPattern Formation and Solitonstext