Kuratowski closure axiomsIn topology and related branches of mathematics, the Kuratowski closure axioms are a set of axioms that can be used to define a topological structure on a set. They are equivalent to the more commonly used open set definition. They were first formalized by Kazimierz Kuratowski, and the idea was further studied by mathematicians such as Wacław Sierpiński and António Monteiro, among others. A similar set of axioms can be used to define a topological structure using only the dual notion of interior operator.
SailA sail is a tensile structure, which is made from fabric or other membrane materials, that uses wind power to propel sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and even sail-powered land vehicles. Sails may be made from a combination of woven materials—including canvas or polyester cloth, laminated membranes or bonded filaments, usually in a three- or four-sided shape. A sail provides propulsive force via a combination of lift and drag, depending on its angle of attack, its angle with respect to the apparent wind.
Déjà vuDéjà vu (ˌdeɪʒɑː_ˈv(j)uː , deʒa vy; "already seen") is a French loanword for the phenomenon of feeling as though one has lived through the present situation before. It is an illusion of memory whereby — despite a strong sense of recollection — the time, place, and context of the "previous" experience are uncertain or impossible. Approximately two-thirds of surveyed populations report experiencing déjà vu at least one time in their lives.
Sail componentsSail components include the features that define a sail's shape and function, plus its constituent parts from which it is manufactured. A sail may be classified in a variety of ways, including by its orientation to the vessel (e.g. fore-and-aft) and its shape, (e.g. (a)symmetrical, triangular, quadrilateral, etc.). Sails are typically constructed out of flexible material that is shaped by various means, while in use, to offer an appropriate airfoil, according to the strength and apparent direction of the wind.
Jamais vuIn psychology, jamais vu (ˌʒæmeɪ_ˈvuː , USˌʒɑːm- , ʒamɛ vy), a French loanword meaning "never seen", is the phenomenon of experiencing a situation that one recognizes in some fashion, but that nonetheless seems novel and unfamiliar. Jamais vu is often described as the opposite of déjà vu. Jamais vu involves a sense of eeriness and the observer’s impression of experiencing something for the first time, despite rationally knowing that they have experienced it before.
Solar sailSolar sails (also known as lightsails, light sails, and photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large surfaces. A number of spaceflight missions to test solar propulsion and navigation have been proposed since the 1980s. The first spacecraft to make use of the technology was IKAROS, launched in 2010. A useful analogy to solar sailing may be a sailing boat; the light exerting a force on the large surface is akin to a sail being blown by the wind.
Crab claw sailThe crab claw sail is a fore-and-aft triangular sail with spars along upper and lower edges. The crab claw sail was developed in Island Southeast Asia some time before 2000 BCE. It went on to be used in many traditional Austronesian cultures in Island Southeast Asia, Micronesia, Island Melanesia, Polynesia, and Madagascar. Due to its extraordinary performance and ease of operation, it has also become very popular in modern sport sailing.
Electric sailAn electric sail (also known as an electric solar wind sail or an E-sail) is a proposed form of spacecraft propulsion using the dynamic pressure of the solar wind as a source of thrust. It creates a "virtual" sail by using small wires to form an electric field that deflects solar wind protons and extracts their momentum. The idea was first conceptualised by Pekka Janhunen in 2006 at the Finnish Meteorological Institute. The electric sail consists of a number of thin, long and conducting tethers which are kept in a high positive potential by an onboard electron gun.
Discrete cosine transformA discrete cosine transform (DCT) expresses a finite sequence of data points in terms of a sum of cosine functions oscillating at different frequencies. The DCT, first proposed by Nasir Ahmed in 1972, is a widely used transformation technique in signal processing and data compression. It is used in most digital media, including (such as JPEG and HEIF), digital video (such as MPEG and H.26x), digital audio (such as Dolby Digital, MP3 and AAC), digital television (such as SDTV, HDTV and VOD), digital radio (such as AAC+ and DAB+), and speech coding (such as AAC-LD, Siren and Opus).
Variational Bayesian methodsVariational Bayesian methods are a family of techniques for approximating intractable integrals arising in Bayesian inference and machine learning. They are typically used in complex statistical models consisting of observed variables (usually termed "data") as well as unknown parameters and latent variables, with various sorts of relationships among the three types of random variables, as might be described by a graphical model. As typical in Bayesian inference, the parameters and latent variables are grouped together as "unobserved variables".
