Re
Re, also known as Ra, was the ancient Egyptian deity embodying the sun and regarded as a creator god who self-generated at the dawn of time and initiated the existence of the cosmos and other divinities.[1][2] As the paramount solar force, Re was depicted as a falcon-headed figure crowned with a sun disk encircled by a cobra, symbolizing his dominion over light, order, and kingship, with pharaohs claiming descent from him to legitimize their rule.[3] His daily voyage across the sky in a divine barque represented the triumph of light over darkness, culminating in a nocturnal descent through the underworld Duat to battle chaos and renew creation at dawn.[4] Re's prominence escalated during the Fifth Dynasty around 2500 BCE, when solar theology permeated state religion, evidenced by pyramid temple complexes dedicated to his worship at sites like Abusir and the integration of his name into royal titulary.[1] Over millennia, he syncretized with other gods, forming composites such as Amun-Re in Thebes, reflecting evolving regional cults while maintaining his core identity as the life-sustaining illuminator whose rays fertilized the earth and sustained ma'at—the principle of cosmic balance.[5] Though his cult waned with the rise of newer deities like Osiris, Re's enduring legacy underpinned Egyptian cosmology, influencing funerary texts like the Book of the Dead, where the deceased aspired to join his eternal solar circuit.[6]Language
Prefix "re-"
The prefix re- originates from Latin re- and red-, denoting "back" or "again," and entered English primarily through Old French re-, where it conveys ideas of return, repetition, or reversal in word formation.[7] This Latin root appears in Proto-Indo-European as re-, linked to backward motion or renewal, influencing verbs and derivatives across Romance languages before adoption into Middle English around the 13th century.[8] In English, re- functions as a derivational morpheme, often prefixed to verbs to indicate an action performed anew or in opposition, as in redo (to do again) or reject (to throw back).[9] Primary usages include repetition, such as rewrite (to write again) or rebuild (to build again), and backward or restorative motion, like return (to turn back) or restore (to place back).[10] In some cases, re- implies undoing or opposition, evident in revoke (to call back) or retract (to draw back), though semantic bleaching occurs in inherited words like receive (originally "take back") or relate (to carry back), where the prefix's force is attenuated over time.[7] The prefix is highly productive in modern English, attachable to most verbs without alteration (e.g., reboot, recharge), but may double before vowels in intensive forms like re-re- (e.g., re-rewrite), a pattern retained from Latin to avoid assimilation.[11] While re- typically preserves its core meanings, occasional intensification emerges, as in resplendent (shining back thoroughly), deriving from Latin re- with adverbial force.[12] English distinguishes re- from homographic forms like the ablative re ("in the matter of," used in legal contexts since circa 1700), but the prefix itself does not overlap with abbreviations such as "Re:" for "regarding."[13] Dictionaries note over 450 English words incorporating re-, spanning domains from regenerate (generate again) to recreational (relating to renewal), underscoring its versatility without implying inherent "dark" connotations in etymology despite varied applications.[14]Abbreviation for "regarding"
"Re" serves as an abbreviation in English-language business correspondence, legal documents, and email subject lines to denote "regarding," "in the matter of," or "with reference to" a specific topic.[15] This usage introduces the primary subject of discussion, as in "Re: Contract Renewal," signaling the focus of the communication.[16] The term derives from the Latin ablative form rē, from rēs ("thing," "matter," or "affair"), literally translating to "in the matter of."[17] This etymology traces back to classical Latin, where in re was employed in legal and formal contexts to specify the affair under consideration, a practice adopted into English by the late Middle Ages and standardized in professional writing by the 18th century.[18][19] In modern email protocols, such as those defined in RFC 5322, "Re:" appears automatically in replies but retains its original semantic role rather than abbreviating "reply," despite common misconceptions; email clients prepend it to maintain continuity with the referenced thread.[20] Legal traditions, including U.S. court filings, continue using "In re" for proceedings not involving specific parties, underscoring the abbreviation's enduring precision in formal address.[15] The convention avoids ambiguity by placing the abbreviation immediately before the descriptive phrase, ensuring clarity in professional exchanges.[16]Science and technology
Chemistry
Rhenium (Re) is a chemical element with atomic number 75 and is classified as a silvery-gray transition metal in group 7 of the periodic table.[21] It possesses one of the highest melting points among metals at 3186 °C and a density of 21.02 g/cm³, making it denser than lead or gold.[22] The element was predicted by Dmitri Mendeleev as "dvi-manganese" based on periodic table trends but was not isolated until 1925, when Walter Noddack, Ida Tacke, and Otto Berg detected it spectroscopically in gadolinite and molybdenite ores at concentrations as low as 10⁻⁷%.[23] Pure rhenium metal was first produced in 1928 through hydrogen reduction of ammonium perrhenate.[24] Rhenium occurs naturally as a mixture of two stable isotopes: rhenium-185 (37.4% abundance) and rhenium-187 (62.6% abundance), with the latter being weakly radioactive with a half-life of 4.35 × 10¹⁰ years.[25] Its average crustal abundance is approximately 1 part per billion, ranking it among the rarest stable elements, typically found in association with molybdenum and copper sulfide ores rather than in concentrated deposits.[26] Global production, primarily as a byproduct of copper and molybdenum refining, totaled around 50 metric tons in 2020, with Chile accounting for over 50% from porphyry copper mines.[27] Chemically, rhenium exhibits oxidation states from −1 to +7, most commonly +4, +5, +6, and +7, forming compounds such as rhenium trioxide (ReO₃), a metallic conductor, and perrhenic acid (HReO₄), from which many salts derive.[22] It resists corrosion and oxidation at high temperatures, though it reacts with strong acids and fuses with alkali metals. In industrial applications, rhenium's high-temperature stability enhances nickel-based superalloys, where additions of 3–6% improve creep resistance in turbine blades and jet engine components, enabling operation at temperatures up to 1200 °C.[28] Additionally, rhenium serves as a catalyst in petroleum reforming processes, converting low-octane naphtha to high-octane gasoline by promoting dehydrogenation reactions, often alloyed with platinum.[29] Emerging uses include radioisotopes like rhenium-186 and rhenium-188 for targeted radiotherapy in cancer treatment, particularly for bone metastases and liver tumors, due to their beta-emission properties and chemical similarity to technetium for conjugation to biomolecules.[30] Rhenium's scarcity and high cost—around $2500 per kilogram in 2023—limit broader adoption, but recycling from superalloy scrap recovers up to 50% of supply.[26] Despite its rarity, rhenium's unique properties ensure its critical role in aerospace and energy sectors, with demand driven by aviation growth.[27]Computing and mathematics
In mathematics, the notation \operatorname{Re}(z) denotes the real part of a complex number z = x + iy, where x is a real number and y is the imaginary part, such that \operatorname{Re}(z) = x.[31] This extraction isolates the component along the real axis in the complex plane, essential for operations in complex analysis, such as computing magnitudes or conjugates.[32] The conjugate of z is \overline{z} = x - iy, and \operatorname{Re}(z) = \frac{z + \overline{z}}{2}.[31] In theoretical computer science, RE refers to the class of recursively enumerable languages or sets, consisting of those decision problems where a Turing machine can verify "yes" instances by halting in acceptance, but may loop indefinitely on "no" instances.[33] Formally, a language L \subseteq \Sigma^* is recursively enumerable if there exists a Turing machine M such that L(M) = L, meaning M accepts exactly the strings in L and either rejects or fails to halt on strings outside L.[34] RE encompasses all computable languages (the recursive class R) but extends to undecidable problems like the halting problem, highlighting the limits of algorithmic decidability; RE-complete problems, such as acceptance by arbitrary Turing machines, capture the hardness within this class.[35] In practical computing, the lowercase "re" commonly designates modules or libraries for regular expressions, such as Python'sre module, which implements pattern matching operations akin to those in Perl for searching, substituting, and parsing strings.[36] Regular expressions define formal patterns over alphabets, generating regular languages—a proper subclass of context-free languages and far below RE in the Chomsky hierarchy—used extensively in text processing tools like grep or sed in Unix-like systems.[37]