42,772 articles
Paras Prasad
Paras N. Prasad, born 1946, is an Indian-born chemist and photonics researcher at the University at Buffalo known for nanophotonics and light-activated nanoparticles for cancer imaging and therapy.
Parasequence
A parasequence is a relatively conformable succession of genetically related beds bounded by marine flooding surfaces, recording one episode of shoreline progradation, introduced by Van Wagoner and colleagues in the 1980s.
Parasitic capacitance
Parasitic capacitance, also known as stray capacitance, is the unintended capacitance that exists between nearby conductors, causing crosstalk, op amp instability, and MOSFET false turn-on in fast circuits.
Paraxial approximation
The paraxial approximation, also called paraxial optics, assumes light travels at small angles to an optical axis so trigonometric functions can be replaced by the angle itself.
Parent hydride
A parent hydride is, in IUPAC chemical nomenclature, an unbranched acyclic or cyclic structure with only hydrogen atoms attached, forming the base name for derivatives like methyl.
Pareto chart
A Pareto chart, or Pareto diagram, is a bar graph ranking causes by frequency or cost, used in quality management as one of the seven basic quality tools.
Pareto distribution
The Pareto distribution, also known as the Bradford distribution, is a heavy-tailed power-law probability distribution derived by Vilfredo Pareto from 1890s income data, underlying the 80/20 rule.
ParFlow
ParFlow (PARallel FLOW) is an open-source, parallel, integrated hydrology model that simulates surface and subsurface water flow with land surface processes, initiated at Lawrence Livermore National Laboratory in the mid-1990s.
Parícutin
Parícutin is a volcano in Michoacán, Mexico, that erupted from 1943 to 1952 after being born in a farmer's cornfield, and no one died directly from it.
Paris green
Paris green, also called emerald green, is a copper arsenite pigment first made in 1814, later used as an insecticide until banned for its toxicity.
Paris law (fatigue)
The Paris law, or Paris–Erdogan law, is an empirical relation stating that fatigue crack growth per load cycle is proportional to a power of the stress-intensity factor range.
Paris–Harrington theorem
The Paris–Harrington theorem, proved by Jeff Paris and Leo Harrington in 1977, strengthens the finite Ramsey theorem and is true but unprovable in Peano Arithmetic.
Parity (mathematics)
Parity (mathematics), also called even and odd numbers, is the property of an integer of being even or odd, depending on whether it is divisible by 2.
Parity (physics)
Parity, or P-symmetry, is the physics operation that inverts all spatial coordinates through the origin; it is conserved by strong and electromagnetic interactions but violated by the weak interaction.
Parity bit
A parity bit, also called a check bit, is an extra binary digit appended to data so the count of 1s is even or odd, detecting single-bit errors.
Parity of a permutation
The parity of a permutation, also called its sign or signature, classifies a permutation as even or odd according to the number of transpositions in any decomposition.
Parity of zero
The parity of zero is the mathematical fact that zero is an even number: it is divisible by 2, and it sits between the odd numbers −1 and 1.
Parity operator
The parity operator, also called the inversion operator, is the quantum-mechanical operator that inverts space, sending every position r to -r. Its only eigenvalues are +1 and -1.
Parity problem in sieve theory
The parity problem in sieve theory is the limitation that classical sieve methods cannot distinguish integers with an odd number of prime factors from integers with an even number.
Parity violation
Parity violation is the failure of a physical process to behave the same as its mirror image, discovered in weak interactions like beta decay in 1957.
Parity-time synthetic photonic lattices
Parity-time synthetic photonic lattices are optical lattices with balanced gain and loss obeying parity-time symmetry, first realized in coupled fiber loops in 2012, showing unidirectional invisibility and exceptional points.
Parity-time-symmetric photonics
Parity-time-symmetric photonics is a branch of optics in which optical gain and loss are arranged so light obeys PT symmetry, enabling real spectra, exceptional points, and single-mode lasers.
Parity–time symmetry
Parity–time symmetry, or PT symmetry, is a symmetry of a physical system under combined spatial reflection (parity) and time reversal, first studied in quantum mechanics and later applied in optics and acoustics.
Park Williams
Park Williams is a hydroclimatologist and Professor of Geography at UCLA whose tree-ring research quantified human-caused warming's role in the southwestern North American megadrought; he received a 2023 MacArthur Fellowship.
Parker spiral
The Parker spiral is the spiral shape the Sun's magnetic field lines take in interplanetary space, predicted by Eugene Parker in 1958 and confirmed by spacecraft in 1962.
Parkes Pulsar Timing Array
The Parkes Pulsar Timing Array, formed in 2004, is an Australian project using the Parkes telescope to time millisecond pulsars in search of nanohertz gravitational waves.
Parking function
A parking function is a sequence of positive integers where cars with those preferences can all park in a line of spots, named by Konheim and Weiss in 1966.
parmbsc1
parmbsc1 is an AMBER-family empirical force field for atomistic molecular dynamics simulations of DNA, published in Nature Methods in 2016 by Modesto Orozco and colleagues.
Parse tree
A parse tree, also called a derivation tree, is an ordered, rooted tree recording how a context-free grammar generates a string, with leaves spelling it out.
Parsec
The parsec, a contraction of "parallax second" coined in 1913, is an astronomical unit of length equal to about 3.26 light-years, the standard distance unit in professional astronomy.