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rhombus lowzi (#) Fr.: losange A quadrilateral having all sides equal and all angles oblique. L.L. rhombus, from Gk. rhombos "rhombus, spinning top," from rhembesthai "to spin, whirl." Lowzi, resembling a lowz "almond." |
rhythm ritm (#) Fr.: rythme 1) An ordered recurrent alternation of strong and weak elements in the
flow of sound and silence in speech; a particular example or form of rhythm. From L. rhythmus "movement in time," from Gk. rhythmos "measured flow or movement, rhythm; proportion, symmetry; arrangement," related to rhein "to flow," from PIE root *sreu- "to flow" Ritm, loan from Fr. |
Ricci scalar marpeli-ye Ricci Fr.: scalaire de Ricci The simplest curvature invariant for a → Riemannian manifold. It is derived from the → Ricci tensor R_{μν} ≡ R^{α}_{μαν} by contracting indices. Taking the trace of the Ricci tensor gives the Ricci scalar: R ≡ R_{μν}g^{μnu;} = R^{μ}_{ν} = R^{αμ}_{αμ}. Also called → scalar curvature. → Ricci tensor; → scalar. |
Ricci tensor tânsor-e Ricci Fr.: tenseur de Ricci A → rank 2, → symmetric tensor R_{μν} that is a contraction of the → Riemann curvature tensor R^{λ}_{μνλ}. More specifically, R_{μν} ≡ Σ (λ) R^{λ}_{μνκ} = R^{λ}_{μνκ}. Closely related to the Ricci tensor is the → Einstein tensor, which plays an important role in the theory of → general relativity. Named after the Italian mathematician Gregorio Ricci-Curbastro (1853-1925); → tensor. |
rich por-, pordâr Fr.: riche Having large amounts of something specified. → metal-rich environment, → rich cluster; → enrich, → enrichment, → richness, → poor. M.E., from O.E. rice "wealthy, powerful" (cf. Du. rijk, Ger. reich "rich"), from PIE base *reg- "move in a straight line," hence, "to direct, rule" (cf. Mod.Pers./Mid.Pers. râst "right, straight;" O.Pers. rāsta- "straight, true," rās- "to be right, straight, true;" Av. rāz- "to direct, put in line, set," razan- "order;" Skt. raj- "to direct, stretch," rjuyant- "walking straight;" Gk. orektos "stretched out;" L. regere "to lead straight, guide, rule," p.p. rectus "right, straight;" Ger. recht; E. right). Por "full, much, very, too much" (Mid.Pers. purr "full;" O.Pers. paru- "much, many;" Av. parav-, pauru-, pouru-, from par- "to fill;" PIE base *pelu- "full," from *pel- "to be full;" cf. Skt. puru- "much, abundant;" Gk. polus "many," plethos "great number, multitude;" O.E. full); pordâr, literally "having much possession," from por + dâr "having, possessor," from dâštan "to have, to possess," → property. |
rich cluster xuše-ye pordâr Fr.: amas riche A → galaxy cluster with a particularly large number of galaxies. |
Richardson cascade peyšâr-e Richardson Fr.: cascade de Richarson Same as → energy cascade Named after L. F. Richardson (1922), Weather Prediction by Numerical Process (Cambridge Univ. Press); → cascade. |
Richardson criterion sanjidâr-e Richardson Fr.: critère de Richardson A condition for the onset of → instability in multilayer fluids which compares the balance between the restoring force of → buoyancy and the destabilizing effect of the → shear. Named after the British meteorologist Lewis Fry Richardson (1881-1953), who first arrived in 1920 to the dimensionless ratio now called → Richardson number. The first formal proof of the criterion, however, came four decades later for → incompressible flows (Miles, J. W. 1961, J. Fluid Mech., 10, 496; Howard, L. N., 1961, J. Fluid Mech., 10, 509). Its extension to → compressible flows was demonstrated subsequently (Chimonas 1970, J. Fluid Mech., 43, 833); → criterion. |
Richardson number adad-e Richardson Fr.: nombre de Richardson A dimensionless number which is used according to the → Richardson criterion to describe the condition for the → stability of a flow in the presence of vertical density stratification. If the → shear flow is characterized by linear variation of velocity and density, with velocities and densities ranging from U_{1} to U_{2} and ρ_{1} to ρ_{2} (ρ_{2}>ρ_{1}), respectively, over a depth H, then the Richardson number is expressed as: Ri = (ρ_{2} - ρ_{1}) gH / ρ_{0} (U_{1} - U_{2})^{2}. If Ri < 0.25, somewhere in the flow turbulence is likely to occur. For Ri > 0.25 the flow is stable. → Richardson criterion; → number. |
richness pordâri Fr.: richesse The property of being very abundant. |
richness class rade-ye pordâri Fr.: classe de richesse A classification of → galaxy clusters into six groups (0 to 5), as in the → Abell catalog. It depends on the number of galaxies in a given cluster that lie within a → magnitude range m_{3} to m_{3+2}, where m_{3} is the magnitude of the 3rd brightest member of the cluster. The first group contains 30-49 galaxies and the last group more than 299 galaxies. |
riddle kervas (#) Fr.: énigme, devinette 1) A question or statement so framed as to exercise one's ingenuity in answering it
or discovering its meaning; conundrum. M.E. redel, redels, from O.E. rædels "riddle; counsel; conjecture; imagination;" cf. O.Fr. riedsal "riddle," O.Sax. radisli, M.Du. raetsel, Du. raadsel, O.H.G. radisle, Ger. Rätsel "riddle." Kervas "riddle, puzzle" [Dehxodâ], Kurd. karvâs "riddle," of unknown origin. |
ridge ruk Fr.: faîte, dorsale A long, narrow elevation of the Earth's surface, generally sharp crested with steep sides, either independently or as part of a larger mountain or hill. See also: → submarine ridge, → wrinkle ridge, → mid-Atlantic ridge. M.E. rigge; O.E. hrycg "spine, back of a man or beast" (cf. O.Fris. hregg, Du. rug, O.H.G. hrukki, Ger. Rücken "the back"). Ruk, from dialectal Tabari ruk "mountain, ridge;" cf. (Dehxodâ) raš "hill." |
Riemann curvature tensor tânsor-e xamidegi-ye Riemann Fr.: tenseur de courbure de Riemann A 4th → rank tensor that characterizes the deviation of the geometry of space from the Euclidean type. The curvature tensor R^{λ}_{μνκ} is defined through the → Christoffel symbols Γ^{λ}_{μν} as follows: R^{λ}_{μνκ} = (∂Γ^{λ}_{μκ})/(∂x^{ν}) - (∂Γ^{λ}_{μν})/(∂x^{κ}) + Γ^{η}_{μκ}Γ^{λ}_{ην} - Γ^{η}_{μν}Γ^{λ}_{ηκ}. → Riemannian geometry; → curvature; → tensor. |
Riemann problem parâse-ye Riemann Fr.: problème de Riemann The combination of a → partial differential equation and a → piecewise constant → initial condition. The Riemann problem is a basic tool in a number of numerical methods for wave propagation problems. The canonical form of the Riemann problem is: ∂u/∂t + ∂f(u)/∂x = 0, x ∈ R, t > 0, u(x,0) = u_{l} if x < 0, and u(x,0) = u_{r} if x > 0 . → Riemann's geometry; → problem. |
Riemann's geometry hendese-ye Riemann Fr.: géométrie de Riemann Same as → Riemannian geometry. → Riemannian; → geometry. |
Riemannian Riemanni (#) Fr.: riemannien Of or pertaining to Georg Friedrich Bernhard Riemann (1826-1866) or his mathematics findings. → Riemannian geometry, → Riemannian manifold, → Riemannian metric, → Riemann problem, → Riemann curvature tensor. After the German mathematician Georg Friedrich Bernhard Riemann (1826-1866), the inventor of the elliptic form of → non-Euclidean geometry, who made important contributions to analysis and differential geometry, some of them paving the way for the later development of → general relativity. |
Riemannian geometry hendese-ye Riemanni Fr.: géométrie riemannienne A → non-Euclidean geometry in which there are no → parallel lines, and the sum of the → angles of a → triangle is always greater than 180°. Riemannian figures can be thought of as figures constructed on a curved surface. The geometry is called elliptic because the section formed by a plane that cuts the curved surface is an ellipse. → Riemannian; → geometry. |
Riemannian manifold baslâ-ye Riemanni Fr.: variété riemannienne A → manifold on which there is a defined → Riemannian metric (Douglas N. Clark, 2000, Dictionary of Analysis, Calculus, and Differential Equations). → Riemannian; → metric. |
Riemannian metric metrik-e Riemanni Fr.: métrique riemannienne A positive-definite inner product, (.,.)_{x}, on T_{x}(M), the tangent space to a manifold M at x, for each x ∈ M, which varies continually with x (Douglas N. Clark, Dictionary of Analysis, Calculus, and Differential Equations). → Riemannian; → metric. |
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