Density of states
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Density of states (DOS) is a property in condensed matter physics that quantifies how closely packed energy levels are in some physical system. It is often expressed as a function g(E) of the internal energy E, or a function g(k) of the wavevector k. It is usually used with electronic energy levels in a solid. In 3 dimensions, for example, the density of states in reciprocal space (k-space) is <math>g(k)=\frac{V}{2\pi^2} k^2 dk<math>, where V is the volume of the solid.
A more precise definition is as follows: g(E) dE is the number of allowed energy levels per unit volume of the material, within the energy range E to E + dE (and equivalently for k).Template:Physics-stub