(generalized) hyperbolic distribution

where K_λ is the modified Bessel function of the second kind.

The kind of generalized hyperbolic distribution depends on the parameter λ:

• λ = 1: hyperbolic distribution
  This function was introduced in finance by Eberlein & Keller (1995).
  
  
  
• λ = - 1/2: the normal inverse Gaussian distribution
  
  The curve is also named (abbreviated) the NIG distribution.
  Barndorff-Nielsen studied the function (1997).