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Partition function (number theory)
The partition function p(n) counts the ways a nonnegative integer can be written as a sum of positive integers, ignoring order; for example, p(4) = 5.
Partition function (statistical mechanics)
The partition function, also called the partition sum, is a normalized weighted sum over a system's possible states in statistical mechanics, whose logarithm yields internal energy, entropy, and free energy.
Partition lattice
The partition lattice is the partially ordered set of all set partitions of a finite set, ordered by refinement, written Πn and counted by Bell numbers.
Partition matroid
A partition matroid is a standard matroid example whose ground set is divided into disjoint parts, with a set independent when it contains at most one element from each part.
Partition numbers
The partition numbers p(n) count the ways a nonnegative integer n can be written as a sum of positive integers, ignoring order; the sequence begins 1, 1, 2, 3, 5, 7.
Partition of a set
A partition of a set, or set partition, is a collection of nonempty, pairwise disjoint subsets, called blocks, whose union is the whole set, with every element in exactly one block.
Partition of unity
A partition of unity is a collection of functions on a space, valued between 0 and 1, that sum to 1 at every point and exist when the space is paracompact.
Parton (particle physics)
A parton is a pointlike constituent of a hadron, a quark, antiquark, or gluon, introduced by Richard Feynman in the late 1960s to explain Bjorken scaling in deep inelastic scattering.
Parton distribution functions
Parton distribution functions, or PDFs, are functions describing how a hadron's momentum and spin are divided among its quark and gluon constituents, and are extracted from collision data.
Parton shower
A parton shower is a Monte Carlo simulation method in particle physics that models sequential quark and gluon emissions from high-energy collisions, producing the jets experiments observe.
Parts-per notation
Parts-per notation is a family of pseudo-units, such as parts per million (ppm) and parts per billion (ppb), for expressing small dimensionless ratios like trace concentrations.
Pascal (unit)
The pascal (Pa) is the SI derived unit of pressure and stress, equal to one newton per square meter; it was named for Blaise Pascal in 1971.
Pascal Schlosser
Pascal Schlosser is a German statistical genomics researcher who leads the Computational Medicine group at the University of Freiburg and holds an adjunct appointment at Johns Hopkins.
Pascal's law
Pascal's law, or Pascal's principle, is a fluid mechanics principle stating that pressure applied to a confined fluid is transmitted undiminished in all directions, underlying hydraulic brakes and jacks.
Pascal's pyramid
Pascal's pyramid, also called Pascal's tetrahedron, is a three-dimensional arrangement of the trinomial coefficients of (x+y+z)^n, the analog of Pascal's triangle, with each entry the sum of the three above it.
Pascal's theorem
Pascal's theorem, also called the mystic hexagram theorem, is a projective geometry result: opposite sides of a hexagon inscribed in a conic meet in three collinear points.
Pascal's triangle
Pascal's triangle, also called the Khayyam or Yang Hui triangle, is a triangular array of binomial coefficients where each number is the sum of the two above it.
Pascale Senellart
Pascale Senellart, born 1972, is a French quantum optician and CNRS research director at Université Paris-Saclay known for semiconductor quantum-dot single-photon sources; she co-founded Quandela and joined the Académie des sciences in 2022.
Paschal Triduum
The Paschal Triduum, or Easter Triduum, is the three-day Christian liturgical period from Holy Thursday evening to Easter Sunday evening commemorating Christ's death and resurrection.
Paschen's law
Paschen's law, or the Paschen curve, states that the breakdown voltage of a gas depends on the product of pressure and gap length, published by Friedrich Paschen in 1889.
Pascua-Lama
Pascua-Lama is a binational gold-silver-copper mining project straddling the Chile-Argentina border in the Andes above 4,500 meters, developed by Barrick Gold and definitively closed in 2020.
Pascual Jordan
Pascual Jordan (1902–1980) was a German theoretical physicist who co-founded matrix mechanics with Born and Heisenberg, pioneered quantum field theory, introduced Jordan algebras, and later served in the Bundestag.
PASEF
PASEF (parallel accumulation–serial fragmentation) is a mass spectrometry acquisition method introduced in 2015 that synchronizes ion mobility with rapid precursor selection, enabling hundreds of fragment spectra per second.
Passerini reaction
The Passerini reaction is a three-component organic reaction in which a carbonyl compound, carboxylic acid, and isocyanide form an α-acyloxy carboxamide; it was discovered in 1921.
Passive infrared sensor
A passive infrared sensor, also known as a PIR sensor or passive infrared detector, detects moving people or warm objects by measuring changes in the infrared radiation they emit.
Passive margin
Passive margin is a tectonic transition between continental and oceanic crust, forming where a continent rifted apart, such as along the Atlantic coast of North America.
Passive radiative cooling
Passive radiative cooling (PRC) is a zero-energy cooling method in which surfaces reflect most sunlight and emit infrared heat through the atmospheric transparency window to space, reducing air-conditioning demand.
Passive sampling
Passive sampling is an analytical chemistry technique where chemicals in air or water move onto a sorbent without pumps or power, giving time-weighted average concentrations.
Passive seismology
Passive seismology is the imaging of Earth's subsurface using natural seismic sources, earthquakes and ambient noise, instead of controlled artificial sources, at much lower cost.
Passive sensing
Passive sensing is the collection of behavioral, physiological, or environmental data through embedded device sensors such as accelerometers and GPS, continuously and without requiring active user participation.