For singularities in differential geometry, see singularity theory. The Gauss-Legendre quadrature rule is not typically used for integrable functions with endpoint singularities. Another general feature of general relativity is the appearance of spacetime boundaries known as singularities. The remarkable behavior of holomorphic functions near essential singularities is described by Picard's Theorem. Sometimes integrals may have two singularities where they are improper. I singularities are gradient shrinking Ricci solitons. The situation becomes easier when dealing with "nonsingular" curves, i.e. those that lack any singularities. Two dimensional terminal singularities are smooth. Berger and Moncrief began a program to analyze generic cosmological singularities. But there are other types of singularities, like cusps. Two dimensional log canonical singularities have been classified by . These shapes often have cusp singularities. The simplest example of singularities are curves that cross themselves. Other than isolated singularities, complex functions of one variable may exhibit other singular behaviour. The following principle of removal of singularities holds for harmonic functions.