differ, a Fortran90 code which is given function values at equally spaced locations, and the order of a derivative, and uses the finite difference method (FDM) to compute a linear combination of these function values which approximated that derivative.
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differ is available in a C version and a C++ version and a Fortran77 version and a Fortran90 version and a MATLAB version and an Octave version and a Python version.
divdif, a Fortran90 code which uses divided differences to compute the polynomial interpolant to a given set of data.