NAME: deriver PURPOSE: (one line) Compute the step-wise derivative of a function of one variable. DESCRIPTION: Calculates the step-wise derivative of a function by computing deltaf / deltax. This requires that f and x be vectors with at least two elements. The uncertainties associated with the dependent variable of the function may be supplied via a keyword, in which case the uncertainties associated with the derivative are returned to the caller via another keyword parameter. CATEGORY: Mathematical CALLING SEQUENCE: deriver, x, f, newx, fprime, [, FERR=in_ferr, FPERR=out_fperr] INPUTS: x : Independent variable. f : Dependent variable. OPTIONAL INPUT PARAMETERS: KEYWORD PARAMETERS: FERR = Uncertainty in f (optional input). FPERR = Uncertainty in fprime (optional output). OUTPUTS: newx : The centered x-values corresponding to fprime. fprime : The computed derivative of the dependent variable. COMMON BLOCKS: SIDE EFFECTS: RESTRICTIONS: PROCEDURE: MODIFICATION HISTORY: Written by Doug Loucks, Lowell Observatory, July, 1993. 1999 Nov 24, Marc W. Buie, Lowell Observatory, changed name to eliminate a naming conflict with DERIVE, an IDL built-in routine.