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Two examples of geometrically frustrated systems in nature. (a) An Ising magnet with antiferromagnetic bonds on a triangular lattice; a bond with unfavorable spins orientations is marked with a wiggly arrow. (b) Five fold and seven disclinations are marked with blue and red shades on a tesellated sphere. Euler theorem requires the presence of at least 12 five fold disclinations to account for the non zero Gaussian curvature of the sphere.
