Editing Omicron (section)

==Use==
In addition to its use as an alphabetic letter, omicron is occasionally used in technical notation,{{citation needed|date=November 2021}} but its use is limited since both upper case and lower case (Ο ο) are indistinguishable from the [[Latin alphabet|Latin letter]] "o" (O o) and difficult to distinguish from the [[Arabic numerals|Arabic numeral]] "zero" (0).

===Mathematics===

The [[Big O notation|big-O]] symbol was introduced by [[Paul Bachmann]] in 1894 and popularized by [[Edmund Landau]] in 1909, originally standing for "order of" ("Ordnung") and being thus a Latin letter, was apparently viewed by [[Donald Knuth]] in 1976<ref name="knuth">{{cite journal|first=Donald|last=Knuth|url=http://www.phil.uu.nl/datastructuren/10-11/knuth_big_omicron.pdf|title=Big Omicron and big Omega and big Theta|journal=SIGACT News|date=April–June 1976|volume=8|issue=2|pages=18–24|doi=10.1145/1008328.1008329|s2cid=5230246|access-date=2021-11-27|archive-date=2021-11-30|archive-url=https://web.archive.org/web/20211130005215/https://www.phil.uu.nl/datastructuren/10-11/knuth_big_omicron.pdf|url-status=dead}}</ref> 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.

===Greek numerals===
{{main|Greek numerals}}
There were several systems for writing [[Greek numerals|numbers 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, in [[Euclid]]'s [[Euclid's Elements|''Elements'']], when various points in a [[geometry|geometric]] 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.{{efn
|name=ordinal_greek_letters|
Greek [[Greek numerals|letters-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 ({{mvar|[[ε]]}}) were shifted from the standard alphabet; 5th place was marked with normal fifth letter [[epsilon]] ({{math|[[ε]]}}). The 6th letter in the conventional alphabet, that normally follows  {{math|[[ε]]}}  is  {{math|[[ζ]]}}  ([[zeta]]) but the ''number'' 6 was represented a revived ancient letter {{mvar|{{prime}}[[ϝ]]}} ([[digamma]]), followed by {{math|{{prime}}[[ζ]]}} which was pushed up from 6th to its ancient position (7th) to represent the number 7. All of the letters after {{math|[[ζ]]}} were likewise shifted up one place, until the second ancient letter [[koppa (letter)|koppa]], ({{math|[[ϙ]]}}), was reached; it fell between {{math|[[Pi (letter)|π]]}} and {{math|[[ρ]]}}. Every letter from [[ρ]] to [[ω]] was shifted ''two'' places past its conventional ordinal position. Last place coming right after [[omega]] ({{math|[[ω]]}}, 800) was [[sampi]] ({{math|ϡ}}) which represented 900. (From that point, the system restarted, with a new tick-mark, at {{math|'''{{big|͵}}'''[[α]]}}. The tick-mark was put in a different place ({{math|'''{{big|͵}}'''[[α]]}} rather than {{math|{{prime}}[[α]]}}) to show that the letter represented a multiple of 1,000 rather than 1.){{Citation needed|date=July 2021}}
}}{{efn
|name=diagrams|
From [[Euclid's Elements|Euclid]] up to the 19th&nbsp;century, mathematical and technical diagrams were habitually marked sequentially with letters (or numbers),{{Citation needed|date=July 2021}} whereas in modern mathematical and scientific diagrams, it is much more common to choose for markers letters that might remind readers of the ''word'' used to describe the item in question.{{Citation needed|date=July 2021}} For example, [[Feynman diagram]]s in [[particle physics]] label the positions of particles with the first letter of their name, either in the Latin or Greek alphabet. So &nbsp;{{math|p}}, {{math|n}}, and {{math|e}}&nbsp;, represent the position on a diagram of a &nbsp;[[proton|<u>p</u>roton]], [[neutron|<u>n</u>eutron]], and [[electron|<u>e</u>lectron]],&nbsp; respectively. The [[neutrino|<u>n</u>eutrino]] is represented by {{mvar|[[ν]]}} (Greek [[ν|"nu"]]), since the Latin letter "{{math|n}}" is reserved for the [[neutron|<u>n</u>eutron]].{{Citation needed|date=July 2021}}
}}

===Astronomy===
Omicron is used to designate the fifteenth star in a constellation group, its ordinal placement an irregular function of both magnitude and position.<ref name="martin">
{{cite book
 |last=Martin |first=Martha Evans
 |year=1907
 |title=The Friendly Stars
 |edition=1st |page=[https://archive.org/details/friendlystars01martgoog/page/n154 135]
 |publisher=[[Harper (publisher)|Harper & Brothers Publishers]]
 |location=New York
 |url=https://archive.org/details/friendlystars01martgoog
 |access-date=8 February 2016
}}
</ref><ref name="wilk">
{{cite book
 |last=Wilk |first=Stephen R.
 |year=2007
 |title=Medusa: Solving the Mystery of the Gorgon
 |edition=1st |page=201
 |publisher=[[Oxford University Press]]
 |location=New York; London
 |isbn=9780199887736
 |url=https://books.google.com/books?id=OnHO4orvz18C&pg=PT201
 |access-date=8 February 2016
}}
</ref> Such stars include [[Omicron Andromedae]], [[Omicron Ceti]], and [[Omicron Persei]].

In [[Ptolemy|Claudius Ptolemy]]'s ({{circa|100–170}}) ''[[Almagest]]'', tables of [[sexagesimal]] numbers &nbsp;{{math|1 ... 59}}&nbsp; are represented in the conventional manner for [[Greek numerals|Greek numbers]]:{{efn
|name=why_omicron_is_available| 
[[Sexagesimal]] [[Greek numerals|Greek numbers]] in the ''[[Almagest]]'' are [[Greek numerals|conventional]]: &nbsp; {{math| 1 2 3 4 5 6 7 8 9 &nbsp;{{=}}&nbsp; ′α  ′β  ′γ  ′δ  ′ε  ′''ϝ''  ′ζ  ′η  ′θ}} &nbsp; and &nbsp; {{math| 10  20  30  40  50 &nbsp;{{=}}&nbsp; ′ι  {{prime}}κ  ′λ  ′μ  ′ν}}&nbsp;. Notice that ancient [[digamma]] ({{mvar|ϝ}}) is used for &nbsp;6&nbsp; instead of [[zeta]] ({{math|ζ}}, which is used for 7)&nbsp;. Adjacent number-letters add, so all the other numbers are made by letter pairs, such as {{math|&nbsp;29  30  31 &nbsp;{{=}}&nbsp; ′κθ  ′λ  ′λα}}&nbsp;. The number 59 ({{math|′νθ}}) is the largest value used in any single number cell in [[sexagesimal]]. That leaves [[Xi (letter)|xi]] ({{math|ξ}}) and the letters following it ({{math|&nbsp;ξ ο π ϙ ρ σ τ υ φ χ ψ ω ϡ&nbsp;}}) free for other use: [[Ptolemy]] picked {{math|&nbsp;′ο&nbsp;}}, which normally was used for {{math|70}}, to mark empty (zero) cells, perhaps because the word for "nothing", {{math|οὐδέν}} starts with an omicron.
}}{{math| ′α  ′β ... ′νη ′νθ }}. Since the letter omicron [which represents {{math|70}} ({{math|′ο}}) in the standard system] is not used in [[sexagesimal]], 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 ({{math|′ο}}) resembles a modern [[Arabic numerals|Hindu-Arabic]] zero&nbsp;({{math|0}}).

===Medicine ===
{{main|SARS-CoV-2 Omicron variant}}
The [[World Health Organization]] (WHO) uses the Greek alphabet to describe [[Variant of concern|variants of concern]] of [[Severe acute respiratory syndrome coronavirus 2|SARS‑CoV‑2]], the virus which causes [[COVID-19]].<ref>{{Cite journal|date=2021-06-09|title=Embrace the WHO's new naming system for coronavirus variants|journal=Nature|language=en|volume=594|issue=7862|pages=149|doi=10.1038/d41586-021-01508-8|pmid=34108702|bibcode=2021Natur.594..149.|s2cid=235395073|doi-access=free}}</ref> On November 26, 2021, Omicron was assigned to the B.1.1.529 [[variant of concern]].<ref>
{{cite web|title=Classification of Omicron (B.1.1.529): SARS-CoV-2 Variant of Concern|url=https://www.who.int/news/item/26-11-2021-classification-of-omicron-(b.1.1.529)-sars-cov-2-variant-of-concern|access-date=26 November 2021|publisher=World Health Organization}}
</ref>