A hierarchical gradient of an unstructured mesh M0 is a sequence of meshes M1;...;Mk such that jMkj is smaller than a given threshold mesh size b. The gradient is well-conditioned if for each i in the range 1 i k, (1) Mi is well-shaped, namely, elements of Mi have a bounded aspect ratio; and (2) Mi is
Gary L. Miller, Dafna Talmor, Shang-Hua Teng