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POLINT

POLINT

Name


    POLINT

Purpose


    Interpolate a set of N points by fitting a polynomial of degree N-1

Explanation


    Adapted from algorithm in Numerical Recipes, Press et al. (1992),
    Section 3.1.

Calling Sequence


    POLINT, xa, ya, x, y, [ dy ]

Inputs


    XA - X Numeric vector, all values must be distinct. The number of
          values in XA should rarely exceed 10 (i.e. a 9th order polynomial)
    YA - Y Numeric vector, same number of elements
    X - Numeric scalar specifying value to be interpolated

Output


    Y - Scalar, interpolated value in (XA,YA) corresponding to X

Optional Output


    DY - Error estimate on Y, scalar

Example


    Find sin(2.5) by polynomial interpolation on sin(indgen(10))
              IDL> xa = indgen(10)
              IDL> ya = sin( xa )
              IDL> polint, xa, ya, 2.5, y ,dy
           
    The above method gives y = .5988 & dy = 3.1e-4 a close
    approximation to the actual sin(2.5) = .5985

Method


    Uses Neville's algorithm to iteratively build up the correct
    polynomial, with each iteration containing one higher order.

Revision History


    Written W. Landsman January, 1992
    Converted to IDL V5.0 W. Landsman September 1997



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