  (abridged) The case can be made for a rather universal stellar IMF form that
can be approximated by a two-part power-law function in the stellar regime.
However, there exists a possible hint for a systematic variation with
metallicity. A picture is emerging according to which the binary properties of
very-low-mass stars (VLMSs) and BDs may be fundamentally different from those
of late-type stars implying the probable existence of a discontinuity in the
IMF, but the surveys also appear to suggest the number of BDs per star to be
independent of the physical conditions of current Galactic star formation.
Star-burst clusters and thus globular cluster may, however, have a much larger
abundance of BDs. Very recent advances have allowed the measurement of the
physical upper stellar mass limit, which also appears to be disconcertingly
robust to variations in metallicity. Furthermore, it now appears that star
clusters may be formed such that the most-massive stars just forming terminate
further star-formation within the particular cluster. Populations formed from
many star clusters, composite populations, would then have steeper IMFs (fewer
massive stars per low-mass star) than the simple populations in the constituent
clusters. A near invariant star-cluster mass function implies the maximal
cluster mass to correlate with the galaxy-wide star-formation rate. This then
leads to the result that the composite-stellar IMFs vary in dependence of
galaxy type, with potentially dramatic implications for theories of galaxy
formation and evolution.
