编辑“︁Ο”︁（章节）

===希腊数字===
{{main|希腊数字}}
在希腊语中，有几种数字书写系统；在古典时代后期，最常见的形式是用omicron（大写或小写）来代表70这个数值。

更为普遍的是，在任何希腊字母标记的列表中，字母omicron都被用来标记第15个序数的位置。因此，例如在[[欧几里得]]的《[[几何原本]]》中，当[[几何学|几何]]图中的各个点用字母标记时，实际上与用数字标记相同，每个字母代表其在标准字母表中的位置数字。{{efn
|name=ordinal_greek_letters|
Greek [[希腊数字|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|&epsilon;]]}}) were shifted from the standard alphabet; 5th place was marked with normal fifth letter [[epsilon]] ({{math|[[Ε|&epsilon;]]}}). The 6th letter in the conventional alphabet, that normally follows  {{math|[[Ε|&epsilon;]]}}  is  {{math|[[Ζ|&zeta;]]}}  ([[zeta]]) but the ''number'' 6 was represented a revived ancient letter {{mvar|&prime;[[Ϝ|ϝ]]}} ([[digamma]]), followed by {{math|&prime;[[Ζ|&zeta;]]}} which was pushed up from 6th to its ancient position (7th) to represent the number 7. All of the letters after {{math|[[Ζ|&zeta;]]}} were likewise shifted up one place, until the second ancient letter [[koppa (letter)|koppa]], ({{math|[[Ϙ|ϙ]]}}), was reached; it fell between {{math|[[Π|&pi;]]}} and {{math|[[Ρ|&rho;]]}}. Ever letter from [[Ρ|&rho;]] to [[Ω|&omega;]] was shifted ''two'' places past its conventional ordinal position. Last place coming right after [[omega]] ({{math|[[Ω|&omega;]]}}, 800) was [[Ͳ|sampi]] ({{math|ϡ}}) which represented 900. (From that point, the system restarted, with a new tick-mark, at {{math|'''{{big|͵}}'''[[Α|α]]}}. The tick-mark was put in a different place ({{math|'''{{big|͵}}'''[[Α|α]]}} rather than {{math|&prime;[[Α|α]]}}) to show that the letter represented a multiple of 1,000 rather than 1.)
}}{{efn
|name=diagrams|
From [[几何原本|Euclid]] up to the 19th&nbsp;century, mathematical and technical diagrams were habitually marked sequentially with letters (or numbers), 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. For example, [[费曼图|Feynman diagram]]s in [[粒子物理學|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;[[質子|<u>p</u>roton]], [[中子|<u>n</u>eutron]], and [[电子|<u>e</u>lectron]],&nbsp; respectively. The [[中微子|<u>n</u>eutrino]] is represented by {{mvar|[[Ν|&nu;]]}} (Greek [[Ν|"nu"]]), since the Latin letter "{{math|n}}" is reserved for the [[中子|<u>n</u>eutron]].
}}