sf

         Compute the structure function of a lightcurve, according to the
         definitions given by Nandikotkur et al., 1997, AIP
         Conf. Proc 410, 1361P for the evenly case and Paltani et
         al., 1997 A&A 327, 539P for the unevenly case.

         time series analysis

         sf,time,rate,lag,sf

         time      : The times at which the time series was measured
         rate      : the corresponding count rates

         none

         delta     : width for the lag rebining in the unevenly case
                     (if not set, is equal to 1e-6)
         logdelta  : delta in logaritmic expression
         evenly    : if set, compute the estimation of the structure
                     function for evenly sample data, otherwise
                     compute the estimation for unevenly sample data
         lagsample : sample of time lags, if not set, all possible
                     lags in the time sample are take as lag sample.
         mincouples: minimum number of couples in a bin of the
                     structure function not to be ignored, if not set,
                     is equal to 1.

         sf        : the "structure function"-values to each time lag
         lag       : sample of time lags

         none

         none

         none

         none

         none

         sf,time,rate,lag,sf,delta=2.

         Version 1.0, 1998.12.21, Sara Benlloch (IAAT)
         Version 1.1, 1999.01.07, lag sample in the unevenly case modificied.
                                  (benlloch@astro.uni-tuebingen.de)
         Version 1.2, 1999.01.26, new keywords; logdelta, lagsample,
                                  mincoupples.

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