Density (disambiguation)

From Wikipedia, the free encyclopedia

Density and dense usually refer to a measure of how much of some entity is "in" a fixed amount of other space. Types of density include:

In physics:

• Density, mass per volume
  • Area density, mass over a (two-dimensional) area
  • Linear density, mass over a (one-dimensional) line
  • Planck density, Planck mass per Planck length
  • Specific gravity, a dimmensionless ratio based on the density of a substance to the density of water.
• Relative density , a measure of density in comparison to the density of something else, often used with respect to liquids
  • Vapour density, a relative density used for gases
• Current density, the ratio of electric current to area
  • Charge density, the electric charge per volume
• Energy density, potential energy per unit volume or mass, depending on context
• Force density, force per unit volume
• Information density, the amount of information compared to many units
• Optical density, the absorbance of an element

In mathematics:

• Dense set and nowhere dense set in topology.
• Schnirelmann density in number theory.
• Natural density (also called asymptotic density) in number theory.
• Lebesgue density in measure theory.
• Probability density function, a function which maps probabilities across the real line and whose integral is 1 (as in, something must always happen).
• Density in graph theory, the fraction of possible edges that exist in a graph.
• Dense order in order theory. In order theory, a subset B of a partially ordered set A is dense in A if for any x < y in A, there is some z in B such that x < z < y. In case the order is a linear order, then B is dense in A in the present sense if and only if B is a dense set with respect to the order topology on A.
• In geometry, dense-in-itself is a set that contains no isolated points.
• In forcing (mathematics), a subset D of a forcing notion (P, ≤ ) is called dense in P if for any p in P there is d in D with d≤p. (Note: since a forcing notion is also a partial order, this usage of the word "dense" conflicts with the definition given above. However, the above notion of "density" is practically never used when talking about forcing notions, so any ambiguities are usually easily resolved from the context.)

In other scientific fields:

• Population density, population per unit area
• Computer storage density, bits (how computers store information) over an amount of area or volume

Density can also refer to:

• Spiritual density, states of being for various planes of existence
• Density (band), an Italian progressive rock band