Identifier     polyint   

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

 Synopsis       polyint, xp , yp , x, y    
                             

 Arguments      Name      I/O       Type           Description
                -----------------------------------------------------
                xp        I   any except string   input x array 
                yp        I   any except string   input y array 
                x         I   any except string   x value to be interpolated
                y         O   any except string   y value corresponding to x

 Returns        y - Scalar, interpolated value in (xp,yp) corresponding to x

 Description    Interpolate a set of N points by fitting a polynomial of 
                degree N-1.
                Adapted from algorithm in Numerical Recipes, Press et al. 
                (1986) Section 3.1 (p82).
                Uses Neville's algorithm to iteratively build up the correct
                polynomial, with each iteration containing one higher order.

 Comment        

 Example   	 Find sin(2.5) by polynomial interpolation on sin(indgen(10))

		 IDL> xp = indgen(10)
		 IDL> yp = sin( xp )
		 IDL> polyint, xp, yp, 2.5, y ,dy
             
                The above method gives  y = .5988 and dy = 3.1e-4  a close
	         approximation to the actual sin(2.5) = .5985

 Dependencies   CALLS: kwd_notset, sap_error
                CALLED FROM: flux_conserv, inter_pol

 Category       ISAP

 Filename       polyint.pro 

 Author         I. R. Khan       

 Version        0.2

 History        0.1 27-08-95  Design and code       --> irk@ipac
                0.2 01-11-95  Revised parameters    --> irk@ipac
                              checking/Error messages

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Copyright (C) 1995, California Institute of Technology.  ;
U.S. Government Sponsorship under NASA Contract NAS7-918 ;
is acknowledged.                                         ;
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