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Sigma (/ ˈ s ɪ ɡ m ə / SIG-mə; [1] uppercase Σ, lowercase σ, lowercase in word-final position ς; Ancient Greek: σίγμα) is the eighteenth letter of the Greek alphabet.In the system of Greek numerals, it has a value of 200.
In the empirical sciences, the so-called three-sigma rule of thumb (or 3 σ rule) expresses a conventional heuristic that nearly all values are taken to lie within three standard deviations of the mean, and thus it is empirically useful to treat 99.7% probability as near certainty.
sigma: summation operator area charge density: coulomb per square meter (C/m 2) electrical conductivity: siemens per meter (S/m) normal stress: pascal (Pa) scattering cross section: barn (10^-28 m^2) surface tension: newton per meter (N/m) tau: torque: newton meter (N⋅m) shear stress: pascal time constant: second (s)
Greek letters are used in mathematics, science, engineering, and other areas where mathematical notation is used as symbols for constants, special functions, and also conventionally for variables representing certain quantities. In these contexts, the capital letters and the small letters represent distinct and unrelated entities.
The standard deviation of the distribution is (sigma). A random variable with a Gaussian distribution is said to be normally distributed , and is called a normal deviate . Normal distributions are important in statistics and are often used in the natural and social sciences to represent real-valued random variables whose distributions are not ...
4. Standard notation for an equivalence relation. 5. In probability and statistics, may specify the probability distribution of a random variable. For example, (,) means that the distribution of the random variable X is standard normal. [2] 6. Notation for proportionality.
"Sigma” is a slang word for “the best” or someone who is a humble alpha male.
The notation was introduced in 1939 by Paul Dirac, [107] though the notation has precursors in Grassmann's use of the notation [φ|ψ] for his inner products nearly 100 years previously. [ 108 ] Bra–ket notation is widespread in quantum mechanics : almost every phenomenon that is explained using quantum mechanics—including a large portion ...