42,772 articles
Perche (unit)
The perche was a French pre-metric unit of length used for measuring land, equal to 3 toises or 18 French feet, though it varied by province.
Perched water table
A perched water table is a local zone of saturation sitting above the regional water table, held there by a low-permeability layer and separated from the main groundwater by unsaturated soil.
Perchlorate
Perchlorate is the anion ClO4−, the conjugate base of perchloric acid, best known as a rocket propellant oxidizer and a drinking water contaminant affecting the thyroid.
Perchloric acid
Perchloric acid (HClO4), also called hydrogen perchlorate, is the strongest common oxyacid of chlorine, a strong acid and powerful oxidizer that becomes explosive when concentrated above 72%.
Perchloroethylene
Perchloroethylene, also called perc, is a nonflammable chlorinated solvent used chiefly in dry cleaning and metal degreasing, the leading US dry cleaning solvent since the 1940s.
Percival Lowell
Percival Lowell (1855–1916) was an American astronomer and businessman from Boston who founded Lowell Observatory in Flagstaff, Arizona, in 1894 and promoted the theory of Martian canals.
Percolation
Percolation is a mathematical model of random media, introduced by Broadbent and Hammersley in 1957, centered on a critical threshold where isolated clusters become system-spanning.
Percolation model
The percolation model is a mathematical and simulation method studying how connected clusters form and grow in random media, with a critical threshold p_c marking the onset of system-spanning clusters.
Percolation theory
Percolation theory is the mathematical study of connectivity in random media, founded by Broadbent and Hammersley in 1957, where an infinite connected cluster first appears at a critical probability.
Percy Alexander MacMahon
Percy Alexander MacMahon (1854–1929) was a British mathematician and retired Royal Artillery officer, the leading British combinatorialist of his era, known for the Master Theorem, plane partitions, and Combinatory Analysis.
Percy Deift
Percy Deift is a South African-born mathematician, Silver Professor at NYU's Courant Institute, known for integrable systems, random matrix theory, and the nonlinear steepest descent method.
Percy Edward Kent
Percy Edward Kent, known from early life as Peter, was a British petroleum exploration geologist who rose to Chief Geologist at BP, helped find the Forties field, and was knighted in 1973.
Percy John Daniell
Percy John Daniell (1889–1946) was a British mathematician, Senior Wrangler at Cambridge in 1909, who created the Daniell integral and anticipated Kolmogorov's extension theorem by a decade.
Percy John Heawood
Percy John Heawood (1861–1955) was a British mathematician at Durham who exposed the flaw in Kempe's 1879 four-color proof, proved the five-color theorem, and gave the Heawood number bound.
Percy Lavon Julian
Percy Lavon Julian, also known as Percy Julian, was an American organic chemist who synthesized physostigmine and low-cost cortisone from soybeans and was elected to the National Academy of Sciences in 1973.
Percy W. Bridgman
Percy Williams Bridgman (1882–1961) was an American experimental physicist at Harvard who won the 1946 Nobel Prize in Physics for high-pressure research, and wrote The Logic of Modern Physics.
Perdita (moon)
Perdita is a small inner moon of Uranus, about 30 km across, discovered in 1999 in archived Voyager 2 images and confirmed by Hubble in 2003.
Peres–Horodecki criterion
The Peres–Horodecki criterion, also called the positive partial transpose (PPT) criterion, is an entanglement test for bipartite quantum states, proposed by Asher Peres in 1996 and proven by the Horodeckis.
Perfect field
A perfect field is a field in which every irreducible polynomial is separable, meaning it has no repeated roots in any field extension; Steinitz introduced the concept in 1910.
Perfect graph
A perfect graph is a graph in which, for every induced subgraph, the chromatic number equals the size of its largest clique, a property introduced by Claude Berge.
Perfect number
A perfect number is a positive integer that equals the sum of its proper divisors, as in 6 = 1 + 2 + 3 and 28.
Perfect numbers
A perfect number is a positive integer equal to the sum of its proper divisors, such as 6 and 28; every even one arises from a Mersenne prime.
Perfect power
A perfect power is a positive integer that can be written as m^k, with m > 1 and k at least 2, such as 4, 8, 9, and 27.
Perfectly matched layer
A perfectly matched layer (PML) is an artificial absorbing layer that lets finite methods like FDTD and FEM simulate unbounded media, introduced by Bérenger in 1994.
Perfluoroalkyl carboxylic acids
Perfluoroalkyl carboxylic acids (PFCAs), also called perfluorocarboxylic acids, are persistent PFAS chemicals with a fully fluorinated carbon chain attached to a carboxylic acid group; PFOA is the best known.
Perfluorobutane
Perfluorobutane, also called perflubutane or decafluorobutane, is a perfluorocarbon gas with formula C₄F₁₀, best known as the core of Sonazoid ultrasound contrast agent, a Cherenkov radiator gas at CERN, and a potent greenhouse gas.
Perfluorobutanesulfonic acid
Perfluorobutanesulfonic acid (PFBS), also called nonafluorobutanesulfonic acid, is a four-carbon short-chain PFAS that replaced PFOS after 2000 and is highly mobile in water with low bioaccumulation.
Perfluorodecanoic acid
Perfluorodecanoic acid (PFDA), also called nonadecafluorodecanoic acid, is a synthetic long-chain perfluoroalkyl carboxylic acid used in stain-proof coatings and identified by ECHA as toxic for reproduction, persistent, and bioaccumulative.
Perfluorohexane
Perfluorohexane (C₆F₁₄) is a colorless, volatile perfluorocarbon liquid, boiling at 56 °C and dissolving large amounts of gas, used in ultrasound contrast nanodroplets, ¹⁹F MRI, and electronics cleaning.
Perfluorohexanesulfonic acid
Perfluorohexanesulfonic acid (PFHxS) is a synthetic six-carbon PFAS formerly used as a surfactant, now regulated globally as a persistent organic pollutant with a human half-life exceeding 7 years.