The multiplicity function for a two state paramagnet, W(n,N), is the number of spin states such that n of the N spins point in the z-direction.
It is primarily used in introductory statistical mechanics and thermodynamics textbooks to explain the microscopic definition of entropy to students.
If the spins are non-interacting, then the multiplicity function counts the number of states which have the same energy in an external magnetic field.
By definition, the entropy S is then given by the natural logarithm of this number:
This thermodynamics-related article is a stub.