IMF

       Compute an N-component power-law logarithmic initial mass function

       The function is normalized so that the total mass distribution
       equals one solar mass.

       psi = IMF( mass, expon,  mass_range )

       mass - mass in units of solar masses (scalar or vector)
               Converted to floating point if necessary
       expon - power law exponent, usually negative, scalar or vector
               The number of values in expon equals the number of different
               power-law components in the IMF
               A Saltpeter IMF has a scalar value of expon = -1.35
       mass_range - vector containing the mass upper and lower limits of the
               IMF and masses where the IMF exponent changes.   The number
               of values in mass_range should be one more than in expon.
               The values in mass_range should be monotonically increasing.

       psi - mass function, number of stars per unit logarithimic mass interval
               evaluated for supplied masses

       The mass spectrum f(m) giving the number of stars per unit mass
       interval is related to psi(m) by  m*f(m) = psi(m).    The normalization
       condition is that the integral of psi(m) between the upper and lower
       mass limit is unity.

       (1) Print the number of stars per unit mass interval at 3 Msun
               for a Salpeter (expon = -1.35) IMF, with a mass range from
               0.1 MSun to 110 Msun.
               IDL> print, imf(3, -1.35, [0.1, 110] ) / 3
       (2) Lequex et al. (1981, A & A 103, 305) describes an IMF with an
               exponent of -0.6 between 0.007 Msun and 1.8 Msun, and an
               exponent of -1.7 between 1.8 Msun and 110 Msun.    Plot
               the mass spectrum f(m)
               IDL> m = [0.01,0.1,indgen(110) + 1 ]  ;Make a mass vector
               IDL> expon = [-0.6, -1.7]       ;Exponent Vector
               IDL> mass_range = [ 0.007, 1.8, 110]    ;Mass range
               IDL> plot_oo, m, imf(m, expon, mass_range ) / m

       IMF first calculates the constants to multiply the power-law
       components such that the IMF is continuous at the intermediate masses,
       and that the total mass integral is one solar mass.  The IMF is then
       calculated for the supplied masses.   Also see Scalo (1986, Fund. of
       Cosmic Physics, 11, 1)

       None

       Written    W. Landsman              August, 1989
       Set masses LE mass_u rather than LT mass_u  August, 1992
       Major rewrite to accept arbitary power-law components   April 1993
       Convert EXPON to float if necessary  W. Landsman     March 1996
       Converted to IDL V5.0   W. Landsman   September 1997
       Remove call to DATATYPE, V5.3 version  W. Landsman  August 2000

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