Zhimin Zhang, Derivative Superconvergent Points in Finite Element Solutions of Harmonic Functions: A Theoretical Justification, Mathematics of Computation, Vol. 71, No. 240 (Oct., 2002), pp. 1421-1430 ...

Lagrange interpolation estimates a function at a reference position x from known values of the function at distinct non-uniformly spaced points x1, ....., xn. Here, the corresponding η-point ...