In the branch of mathematics called potential theory, a quadrature domain in two dimensional real Euclidean space is a domain D together with a finite subset {z1, …, zk} of D such that, for every function u harmonic and integrable over D with respect to area measure, the integral of u with respect to this measure is given by a "quadrature formula"; that is, ∬ D u d x d y = ∑ j = 1 k c j u, {\displaystyle \iint _{D}u\,dxdy=\sum _{j=1}^{k}c_{j}u,} where the cj are nonzero complex constants independent of u. Wikipedia