| 释义 |
sigma|ˈsɪgmə|
[L. sigma, Gr. σίγµα, the 18th letter of the Greek alphabet.]
1. The name of the Greek letter σ, σ, ς, the equivalent of the English S, s, in its uncial form having the shape of C.
1607Topsell Four-f. Beasts 290 Written with Iota and simple Sigma. 1736Ainsworth Lat. Dict. s.v. S, Which opinion is made more probable by the small form of the Greek sigma. 1833Penny Cycl. I. 385/2 The difference between the shin.., and the Greek sigma..depends solely upon the altered position. 1859Millard tr. Kühner's Gr. Gram. 4 The Sigma (σ), at the end of a word, takes the form ς. 1876S. Birch Rede Lect. on Egypt 32 The final sigma of the Greeks is represented by the Egyptian sibilant.
2. Something having the form of S or C.
1788Gibbon Decl. & F. liii. V. 481 The square before the sigma [= semicircular portico] was decorated with a fountain. 1877Coues & Allen N. Amer. Rod. 32 The loops of enamel..do form a sort of sigma [etc.; see sigmodont].
3. Physics and Chem.
a. [After S 5.] Used to designate electrons, orbitals, molecular states, etc., possessing zero angular momentum about an internuclear axis; sigma- (or σ-) bond, a bond formed by a σ-orbital.
Usu. written σ when it refers to one electron and σ when it refers to a molecule as a whole.
1929, etc. [see pi n. 3]. 1939J. W. T. Spinks tr. Herzberg's Diatomic Molecules v. 260 For multiplet σ states the character positive-negative depends on whether K..is even or odd. 1952L. N. Ferguson Electron Structures of Org. Molecules ii. 21 In butadiene, each carbon atom forms three σ bonds. 1963W. J. Moore Physical Chem. (ed. 4) xiv. 600 In the electronic excitation an electron is removed from a π orbital and placed in a σ orbital. 1964R. G. Parr Quantum Theory of Molec. Electronic Structure iii. 41 It is supposed that somehow the effect of the others, the sigma electrons, can be lumped into the Hamiltonian for the pi electrons. 1966Phillips & Williams Inorg. Chem. II. xxviii. 341 The vinyl anion, C2H3-, binds directly to cobalt (III) of vitamin B12 as a simple σ-bonded ligand. 1972DePuy & Chapman Molec. Reactions & Photochem. vi. 103 The numbers refer to the atoms at either end of the sigma bond which is thought of as moving. 1978P. W. Atkins Physical Chem. xv. 471 Thus an oxygen molecule, which..has two π*-electrons, could be in either a σ state (the electrons orbiting in opposite directions) or in a Δ state (the electrons orbiting in the same sense around the bond).
b. Particle Physics. Used, usu. attrib., to denote any of a triplet of hyperons (and their antiparticles) having an average mass of approximately 1190 MeV (2340 times that of the electron), a spin of ½, zero hypercharge, and unit isospin, and which on decaying usu. produce a nucleon and a pion (if charged) or a lambda particle and a photon (if neutral). Freq. written as σ.
1954Gell-Mann & Pais in Proc. Glasgow Conf. Nuclear & Pleson Physics (1955) 347 The..apparent existence of both a positive and a negative hyperon which we shall call σ+ with the decay schemes σ{pm} → N + π{pm} + (∼115 MeV), σ{pm} → P + π° + (∼115 MeV). 1955Nuovo Cimento II. 824 An event, interpreted as the disappearance of a charged σ-hyperon in flight, has been observed..in the Brookhaven cosmotron. 1961W. S. C. Williams Introd. Elementary Particles xii. 298 Any subsequent σ decays in flight. 1963K. W. Ford World of Elementary Particles vi. 179 (caption) A negative pion..collides with a proton and produces two strange particles, a neutral sigma and a neutral kaon... The sigma particle lives too short a time to move a measurable distance.., decaying almost at once into a lambda and a photon. 1976Sci. Amer. Jan. 46/2 The baryons with the lowest mass are those with a spin of 1/2. There are eight of them: two nucleons (ν).., a lambda particle (λ).., three sigma particles (σ)..and two cascade particles (ξ).
4. Biochem. ellipt. for sigma factor, sense 6 below.
1970New Scientist 23 July 176/1 Soon after infection..the phage makes its own sigma, which redirects the host core enzyme to start making a different set of phage proteins.
5. Statistics. A standard deviation: used in the singular as if the name of a unit.
1978N. R. Ullman Elem. Statistics iii. 68 The difference is 0·5 inches, or, since σ = 0·29..1·72 standard deviations apart or 1·72 σ (1·72 sigma) apart. Ibid. 69 You can express values in terms of sigma or in terms of ‘so many sigma units from the mean’. 1979Nature 29 Mar. 411/1 On each radiocarbon analysis, one sigma counting errors are given.
6. Comb., as sigma factor Biochem., a component of RNA polymerase which determines where transcription begins.
[1969R. R. Burgess et al. in Nature 4 Jan. 44/2 GG enzyme contains, in addition, two extra bands which we shall designate σ and τ. Ibid. 46/1 The presence of the stimulating factor, σ, greatly enhances the amount of RNA synthesis. Ibid. 46/2 σ and similar factors could..act as positive control elements regulating the amount of synthesis of different classes of RNA.] 1969Times 9 May 12/6 The sigma factor helps to specify which genes are expressed. 1970Nature 29 Aug. 886/1 Today sigma factor proteins..are central to all attempts to explain the positive control of gene expression. 1976Proc. Nat. Acad. Sci. LXXIII. 3961 (heading) Purification and characterization of a putative sigma factor from Chlamydomonas reinhardi. Ibid. 4405 (heading) Induction of sigma factor synthesis in Escherichia coli by the N gene product of bacteriophage lambda.
Hence ˈsigmaspire (see quot.). ˈsigmate a., having the form of a sigma. ˈsigmate v. trans., to add a sigma or s to (a word, stem, etc.); hence sigmated ppl. a. sigmatic |sɪgˈmætɪk| a., characterized by the addition of sigma or s to the stem. sigˈmation, the addition of s to a word, etc. ˈsigmatism, ‖ sigmaˈtismus, (a) the marked use or repetition of s; an instance of this; (b) defective articulation of sibilants. ˈsigmatize v. [cf. late Gr. σιγµατίζειν] trans., to mark with the letter s.
1887Encycl. Brit. XXII. 417/2 (Sponges), One of the simplest forms [of microscleres] is the *sigmaspire..; it looks like the letter C or S, according to the direction in which it is viewed, its actual form being that of a single turn of a cylindrical spiral.
1887Amer. Naturalist XXI. 937 [Sponges] with *sigmate flesh-spicules.
1849T. K. Arnold First Gr. Bk. 5 The root of the future is got from the root of the present (or infinitive) by *sigmating it.
Ibid. 22 The first Aorist of the Active is formed by adding ᾰ to the *sigmated root..and prefixing the augment. 1889N. & Q. 7th Ser. VIII. 216/2 The question of the plural treatment, or otherwise, of some sigmated words is fair matter for discussion.
1888King & Cookson Sounds & Infl. Gr. & Lat. xv. 444 The inflexions of the *sigmatic aorist. 1897Trans. Philol. Soc. 233 We have also a subjunctive of the sigmatic aorist.
1889N. & Q. 7th Ser. VII. 142/1 This fondness for pluralizing..is constantly showing itself both in a purely senseless *sigmation and in a duplication of the plural ending.
1888A. H. Buck Reference Handbk. Med. Sci. VI. 617/1 Lisping, or *Sigmatism, is the most common form of stammering. It consists of giving s a wrong sound, usually that of th, by carrying the tip of the tongue too far forward, so as to touch the upper teeth. 1889Classical Rev. III. 270 The sigmatism is quite Ovidian. 1891Athenæum 21 Feb. 246/2 Sophocles, ‘O.T.’ 425, should have been quoted as the most remarkable, and at the same time effective, example of sigmatism in Greek tragedy. 1933S. M. Stinchfield Speech Disorders iv. 76 The boys led in the number of cases of sigmatism, stuttering.., deafness and speech defects as a whole. 1957Dental Practitioner VII. 220/2 Speech may be ‘thick’, which I tend to associate with the large tongue and the lateral sigmatism which suggests tongue behaviour. 1965W. R. Brain Speech Disorders 154 Various disorders of the production of s (sigmatism) have been recognized.
1887Q. Rev. Oct. 369 There are three inseparable necessities which may be remembered by a *sigmatismus—site, soil, and sympathy.
1654H. L'Estrange Chas. I (1655) 145 Mr. Prynne..was..to be stigmatized, or if you will *sigmatized, on both Cheeks with the letter S for a Schismatick. |