FUNCTION VDHAN,N
;
;+
; NAME:
;	VDHAN
;
; PURPOSE:
;	This function computes the "an" coefficient in equation 24 of van de 
;	Hulst's paper
;
; CATEGORY:
;	LASCO DATA_ANALYSIS
;
; CALLING SEQUENCE:
;	VDHAN,N
;
; INPUTS:
;	N:	The index of the term
;
; OUTPUTS:
;	This function returns the an coefficient.
;
; RESTRICTIONS:
;	The index must be positive and less than 34.
;
; PROCEDURE:
;	The procedure computes the an coefficient in equations 24a and 24b of 
;	van de Hulst, ;	Bull Astron Institutes Netherlands, vol XI, 2 Feb 1950, 
;	pp143.  The definition of the coefficient is given in equation 25:
;	
;	an = pi * n! / ((n/2)!)^2 / 2^(n+1)
;
; EXAMPLE:
;
; MODIFICATION HISTORY:
; 	Written by:	RA Howard, NRL, 27 Nov 1998
;
;	@(#)vdhan.pro	1.4 11/02/01 LASCO IDL LIBRARY
;-
   an = 0
   IF (n GE 0) AND (n LT 34.5)  THEN BEGIN
;      an = !pi*(GAMMA(n+1)/(GAMMA(0.5*n+1)^2))/2^(n+1)
;      alog(2) = .69314718
      an = DOUBLE(!pi)*EXP(LNGAMMA(n+1)-2*LNGAMMA(0.5*n+1)-(n+1)*.69314718)
   ENDIF ELSE BEGIN
      PRINT,'ERROR VDHAN: Exponent outside range (0,34.5) = ',n
   ENDELSE
RETURN,an
END