Purpose

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  Evaluate the cumulative distribution function of a lognormal
  distribution.
  The lognormal distribution PDF is defined as
  P(x) = [sqrt(2 * pi) * sigma * x]^(-1) *
        exp[ -ln(x / a)^2 / (2 * sigma)^2]

Inputs

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  x: The abcissa values at which to compute the CDF
  a: The median of the distribution (see formula above)
  sigma: The width of the distribution (see formula above)

Keyword Parameters

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  param: An optional 2-element array, listing [a, sigma]. This is
        provided for convenience, as some functions (e.g., ksone in
        the IDL Astronomy user's library) prefer to pass
        parameters this way.

Outputs

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  EDF(x)

Modification History

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  July 2011: Written by Chris Beaumont