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superconvergence; external approximation; pointwise error estimate; finite element subspaces; orthogonal projections; ordinary differential operators
A pointwise error estimate and an estimate in norm are obtained for a class of external methods approximating boundary value problems. Dependence of a superconvergence phenomenon on the external approximation method is studied. In this general framework, superconvergence at the knot points for piecewise polynomial external methods is established.
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