represents: one of the Gegenbauer functions in analytic number theory. the Dickman-de Bruijn function the radius in a polar, cylindrical, or spherical coordinate system the correlation coefficient in statistics the radius of convergence in real analysis the sensitivity to interest rate in mathematical finance density (mass or charge per unit volume; may be replaced by the capital form of the Latin letter D) resistivity the shape and reshape operators in APL (in the form ⍴) the rename operator in relational algebra the plastic number Σσς (sigma) Σ represents: the summation operator the covariance matrix the set of terminal symbols in a formal grammar represents: Stefan–Boltzmann constant in blackbody radiation the divisor function in number theory the real part of the complex variable in analytic number theory the sign of a permutation in the theory of finite groups the population standard deviation, a measure of spread in probability and statistics a type of covalent bond in chemistry (sigma bond) the selection operator in relational algebra stress in mechanics electrical conductivity area density nuclear cross section surface charge density for microparticles standard deviation of a random variable in statistics Ττ (tau) represents: torque, the net rotational force in mechanics the elementary tau lepton in particle physics a mean lifetime, of an exponential decay or spontaneous emission process the time constant of any device, such as an RC circuit proper time in relativity one turn: the constant ratio of a circle's circumference to its radius, with value (6.283...). Kendall tau rank correlation coefficient, a measure of rank correlation in statistics Ramanujan's tau function in number theory shear stress in continuum mechanics a type variable in type theories, such as the simply typed lambda calculus path tortuosity in reservoir engineering in topology, a given topology the tau in biochemistry, a protein associated to microtubules the number of divisors of highly composite numbers precision (), the reciprocal of variance, in statistics ϒυ (upsilon) ϒ (U+03D2) represents: the upsilon meson Φφ (phi) Φ represents: the work function in physics; the energy required by a photon to remove an electron from the surface of a metal magnetic flux or electric flux the cumulative distribution function of the normal distribution in statistics phenyl functional group in organic chemistry (pseudoelement symbol) the reciprocal of the golden ratio (represented by , below), also represented as the value of the integration of information in a system (based on integrated information theory)