2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/148614We notice some beautiful geometrical defects found in liquid crystals, and explain them by imposing a constraint. We study the way constraints can occur, and introduce the concept of massive fields. We develop the theory of magnetic field expulsion in superconductors as an example. We notice strong analogies with the formation of grain boundaries in crystals, and realize that we do not understand crystals very deeplyLectures Given at the Santa Fe Complexity Institute Summer School, 1991. Figures addedCondensed Mattertext