In addition to its use as an alphabetic letter, omicron is occasionally used in technical notation,[citation needed] but its use is limited since both upper case and lower case (Ο ο) are indistinguishable from theLatin letter "o" (O o) and difficult to distinguish from theArabic numeral "zero" (0).
Thebig-O symbol was introduced byPaul Bachmann in 1894 and popularized byEdmund Landau in 1909, originally standing for "order of" ("Ordnung") and being thus a Latin letter, was apparently viewed byDonald Knuth in 1976[3] as a capital Omicron, probably in reference to his definition of the symbol (capital)Omega. Neither Bachmann nor Landau ever call it "Omicron", and the word "Omicron" appears just once in the title of Knuth's paper.
There were several systems for writingnumbers in Greek; the most common form used in late classical era used omicron (either upper or lower case) to represent the value 70.
More generally, the letter omicron is used to mark the fifteenth ordinal position in any Greek-alphabet marked list. So, for example, inEuclid'sElements, when various points in ageometric diagram are marked with letters, it is effectively the same as marking them with numbers, each letter representing the number of its place in the standard alphabet.[a][b]
Omicron is used to designate the fifteenth star in a constellation group, its ordinal placement an irregular function of both magnitude and position.[4][5] Such stars includeOmicron Andromedae,Omicron Ceti (Mira), andOmicron Persei.
InClaudius Ptolemy's (c. 100–170)Almagest, tables ofsexagesimal numbers 1 ... 59 are represented in the conventional manner forGreek numbers:[c] ′α ′β ... ′νη ′νθ. Since the letter omicron [which represents70 (′ο) in the standard system] is not used insexagesimal, it is repurposed to represent an empty number cell. In some copies, zero cells were just left blank (nothing there, value is zero), but to avoid copying errors, positively marking a zero cell with omicron was preferred, for the same reason that blank cells in modern tables are sometimes filled-in with a long dash (—). Both an omicron and a dash imply that"this is not a mistake, the cell is actually supposed to be empty." By coincidence, the ancient zero-value omicron (′ο) resembles a modernHindu-Arabic zero (0).
Detail from a fifth-century BCE inscription ofDraco's law on homicide, showing the use ofO rather thanΩ in the phrase "ΠΡΟΤΟΣ ΑΧΣΟΝ" (πρώτος ἄξων, "first axon")
In the earliest Greek inscriptions, only five vowel lettersAEIOY were used. Vowel length was undifferentiated, withO representing both the short vowel /o/ and the long vowels /o:/ and /ɔː/.[8](p 19) Later, in classical Attic Greek orthography, the three vowels were represented differently, withO representing short /o/, the new letterΩ representing long /ɔː/, and the so-called "spurious diphthong"OY representing long /o:/.[8](pp 56, 71)
Although the Greeks took the characterO from the Phoenician letter`ayin, they did not borrow its Phoenician name. Instead, the name of the letterO in classical Attic times was simply the long version of its characteric sound:οὖ (pronounced /o:/) (that ofΩ was likewiseὦ).[9][d] By the second and third centuries AD, distinctions between long and short vowels began to disappear in pronunciation, leading to confusion betweenO andΩ in spelling. It was at this time that the new names ofὂ μικρόν ("small O") forOὦ μέγα ("great O") forΩ were introduced.[9]
During the early outbreak of the Omicron variant of COVID-19, many people unfamiliar with the entire Greek alphabet (or simply lacking the ability to pronounce or sound out words usingphonetics) mispronounced Omicron as "Omnicron" due to the unfamiliarity of the letter, and the use of the prefix "Omni-" in many words.[11][12]
These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style:[14]
U+1D6B6𝚶MATHEMATICAL BOLD CAPITAL OMICRON
U+1D6D0𝛐MATHEMATICAL BOLD SMALL OMICRON
U+1D6F0𝛰MATHEMATICAL ITALIC CAPITAL OMICRON
U+1D70A𝜊MATHEMATICAL ITALIC SMALL OMICRON
U+1D72A𝜪MATHEMATICAL BOLD ITALIC CAPITAL OMICRON
U+1D744𝝄MATHEMATICAL BOLD ITALIC SMALL OMICRON
U+1D764𝝤MATHEMATICAL SANS-SERIF BOLD CAPITAL OMICRON
U+1D77E𝝾MATHEMATICAL SANS-SERIF BOLD SMALL OMICRON
U+1D79E𝞞MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL OMICRON
U+1D7B8𝞸MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL OMICRON
^Greekletters-as-numbers used an older Greek alphabet with three more otherwise unused letters, two of them re‑instated in their old locations, early in the alphabet. So positions higher than 5th place (ε) were shifted from the standard alphabet; 5th place was marked with normal fifth letterepsilon (ε). The 6th letter in the conventional alphabet, that normally followsε isζ (zeta) but thenumber 6 was represented a revived ancient letter′ϝ (digamma), followed by′ζ which was pushed up from 6th to its ancient position (7th) to represent the number 7. All of the letters afterζ were likewise shifted up one place, until the second ancient letterkoppa, (ϙ), was reached; it fell betweenπ andρ. Every letter fromρ toω was shiftedtwo places past its conventional ordinal position. Last place coming right afteromega (ω, 800) wassampi (ϡ) which represented 900. (From that point, the system restarted, with a new tick-mark, at͵α. The tick-mark was put in a different place (͵α rather than′α) to show that the letter represented a multiple of 1,000 rather than 1.)[citation needed]
^FromEuclid up to the 19th century, mathematical and technical diagrams were habitually marked sequentially with letters (or numbers),[citation needed] whereas in modern mathematical and scientific diagrams, it is much more common to choose for markers letters that might remind readers of theword used to describe the item in question.[citation needed] For example,Feynman diagrams inparticle physics label the positions of particles with the first letter of their name, either in the Latin or Greek alphabet. So p,n, ande , represent the position on a diagram of a proton,neutron, andelectron, respectively. Theneutrino is represented byν (Greek"nu"), since the Latin letter "n" is reserved for theneutron.[citation needed]
^SexagesimalGreek numbers in theAlmagest areconventional: 1 2 3 4 5 6 7 8 9 = ′α ′β ′γ ′δ ′ε ′ϝ ′ζ ′η ′θ and 10 20 30 40 50 = ′ι′κ ′λ ′μ ′ν . Notice that ancientdigamma (ϝ) is used for 6 instead ofzeta (ζ, which is used for 7) . Adjacent number-letters add, so all the other numbers are made by letter pairs, such as 29 30 31 = ′κθ ′λ ′λα . The number 59 (′νθ) is the largest value used in any single number cell insexagesimal. That leavesxi (ξ) and the letters following it ( ξ ο π ϙ ρ σ τ υ φ χ ψ ω ϡ ) free for other use:Ptolemy picked ′ο , which normally was used for70, to mark empty (zero) cells, perhaps because the word for "nothing",οὐδέν starts with an omicron.
^This is confirmed by the text of the so-calledLetter Tragedy of the fifth-century BCE comic poetCallias, and also by a passage in Plato'sCratylus, whereSocrates states:
[W]hen we speak of the letters of the alphabet, you know, we speak their names, not merely the letters themselves, except in the case of four:E,Y,O, andΩ.[10]
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