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  This function calculates the log-likelihood of the data, under a
  powerlaw model. The powerlaw distribution is given by
  f(x) = (alpha - 1) (xmin)^(alpha - 1) x^(-alpha)



Calling Sequence

  result = pl_like([params, derivs, data = data, alpha = alpha, xmin
                  = xmin])

Optional Inputs

  params: A two element vector specifying [alpha, xmin].
  derivs: A named variable to hold the partial derivative of the
          log-likelihood with respect to alpha and xmin.

Keyword Parameters

  data: A vector of data values, assumed to be >= xmin
  alpha: Another way to specify alpha. This takes precedence over any
        variable stored in params[0].
  xmin: Another way to specify xmin. This takes precedence over any
        variable stored in params[1].


  Ln(Product( f(data_i) ) )


  To maximize speed, the function does not check for values of data <
  xmin. However, the output is useless when any data are < xmin.

Modification History

  June 2009 Written by Chris Beaumont

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