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This article is about the Greek letter. For other uses, see Epsilon (disambiguation).

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**Epsilon** (uppercase **Ε**, lowercase **ɛ** or lunate **ϵ**; Greek: Έψιλον) is the fifth letter of the Greek alphabet, corresponding phonetically to a close-mid front unrounded vowel /e/. In the system of Greek numerals it has the value five. It was derived from the Phoenician letter He . Letters that arose from epsilon include the Roman E, Ë and Ɛ, and Cyrillic Е, È, Ё, Є and Э.

The name of the letter was originally εἶ (Ancient Greek: [êː]), but the name was changed to ἒ ψιλόν (*e psilon* "simple e") in the Middle Ages to distinguish the letter from the digraph αι, a former diphthong that had come to be pronounced the same as epsilon.

In essence, the uppercase form of epsilon looks identical to Latin E. The lowercase version has two typographical variants, both inherited from medieval Greek handwriting. One, the most common in modern typography and inherited from medieval minuscule, looks like a reversed "3". The other, also known as lunate or uncial epsilon and inherited from earlier uncial writing,^{[1]}^{[2]} looks like a semicircle crossed by a horizontal bar. While in normal typography these are just alternative font variants, they may have different meanings as mathematical symbols. Computer systems therefore offer distinct encodings for them.^{[1]} In Unicode, the character U+03F5 "Greek lunate epsilon symbol" (ϵ) is provided specifically for the lunate form. In TeX, `\epsilon`

() denotes the lunate form, while `\varepsilon`

() denotes the inverted-3 form.

There is also a Latin epsilon or "open e", which looks similar to the Greek lowercase epsilon. It is encoded in Unicode as U+025B ("Latin small-letter open e", ɛ) and U+0190 ("Latin capital-letter open e", Ɛ) and is used as an IPA phonetic symbol. The lunate or uncial epsilon has also provided inspiration for the euro sign (€).

The lunate epsilon (ϵ) is not to be confused with the set membership symbol (∈); nor should the Latin uppercase epsilon (Ɛ) be confused with the Greek uppercase sigma (Σ).

The letter Ε was taken over from the Phoenician letter He () when Greeks first adopted alphabetic writing. In archaic Greek writing, its shape is often still identical to that of the Phoenician letter. Like other Greek letters, it could face either leftward or rightward (), depending on the current writing direction, but, just like in Phoenician, the horizontal bars always faced in the direction of writing. Archaic writing often preserves the Phoenician form with a vertical stem extending slightly below the lowest horizontal bar. In the classical era, through the influence of more cursive writing styles, the shape was simplified to the current E glyph.^{[3]}

While the original pronunciation of the Phoenician letter *He* was [h], the earliest Greek sound value of Ε was determined by the vowel occurring in the Phoenician letter name, which made it a natural choice for being reinterpreted from a consonant symbol to a vowel symbol denoting an [e] sound.^{[4]} Besides its classical Greek sound value, the short /e/ phoneme, it could initially also be used for other [e]-like sounds. For instance, in early Attic before c.500 B.C., it was used also both for the long, open /ɛː/, and for the long close /eː/. In the former role, it was later replaced in the classic Greek alphabet by Eta (Η), which was taken over from eastern Ionic alphabets, while in the latter role it was replaced by the digraph spelling ΕΙ.

Some dialects used yet other ways of distinguishing between various e-like sounds.

In Corinth, the normal function of Ε to denote /e/ and /ɛː/ was taken by a glyph resembling a pointed B (), while Ε was used only for long close /eː/.^{[5]} The letter Beta, in turn, took the deviant shape .

In Sicyon, a variant glyph resembling an X () was used in the same function as Corinthian .^{[6]}

In Thespiai (Boeotia), a special letter form consisting of a vertical stem with a single rightward-pointing horizontal bar () was used for what was probably a raised variant of /e/ in pre-vocalic environments.^{[1]}^{[7]} This tack glyph was used elsewhere also as a form of "Heta", i.e. for the sound /h/.

After the establishment of the canonical classic Greek alphabet^{[clarify]}, new glyph variants for Ε were introduced through handwriting. In the uncial script (used for literary papyrus manuscripts in late antiquity and then in early medieval vellum codices), the "lunate" shape () became predominant. In cursive handwriting, a large number of shorthand glyphs came to be used, where the cross-bar and the curved stroke were linked in various ways.^{[8]} Some of them resembled a modern lowercase Latin "e", some a "6" with a connecting stroke to the next letter starting from the middle, and some a combination of two small "c"-like curves. Several of these shapes were later taken over into minuscule book hand. Of the various minuscule letter shapes, the inverted-3 form became the basis for lower-case Epsilon in Greek typography during the modern era.

Uncial | Uncial variants | Cursive variants | Minuscule | Minuscule with ligatures |
---|---|---|---|---|

In the International Phonetic Alphabet, the Latin epsilon /ɛ/ represents open-mid front unrounded vowel, as in the English word "pet" /ˈpɛt/.

The uppercase Epsilon is not commonly used outside of the Greek language because of its similarity to the Latin letter E.

The Greek lowercase epsilon **ε**, the lunate epsilon symbol **ϵ**, or the Latin lowercase epsilon **ɛ** (see above) is used as the symbol for:

- In mathematics (in particular calculus), an arbitrarily small positive quantity is commonly denoted ε; see (ε, δ)-definition of limit.
- By analogy with this, the late mathematician Paul Erdős also used the term "epsilons" to refer to children (Hoffman 1998, p. 4).

- In mathematics, Hilbert introduced epsilon terms as an extension to first order logic; see epsilon calculus.
- In mathematics, the Levi-Civita symbol.
- In mathematics, to represent the dual numbers:
*a*+*bε*, with*ε*^{2}= 0 and*ε*≠ 0. - In mathematics, sometimes used to denote the Heaviside step function.
^{[citation needed]} - In set theory, the epsilon numbers are ordinal numbers that satisfy the fixed point ε = ω
^{ε}. The first epsilon number, ε_{0}, is the limit ordinal of the set {ω, ω^{ω}, ω^{ωω}, ...}. - In computing, the precision of a numeric data type and floating-point machine epsilon.
- In computer science, the empty string, though different writers use a variety of other symbols for the empty string as well, usually the lower-case Greek letter lambda (λ).
- In physics, the permittivity of a medium.
- In physics and electronics, the emf of a circuit
- In physics, the strain of a material (a ratio of extensions).
- In automata theory, a transition that involves no shifting of an input symbol.
- In astronomy, the fifth-brightest star in a constellation (see Bayer designation).
- In astronomy, Epsilon is the name for Uranus' most distant and most visible ring.
- In planetary science, ε is the denotion of axial tilt.
- In chemistry, the molar extinction coefficient of a chromophore.
- In economics, ε refers to elasticity.
- In statistics, to refer to error terms.
- In agronomy, to represent the "photosynthetic efficiency" of a particular plant or crop.

- Greek Epsilon

Character | Ε | ε | ϵ | ϶ | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | GREEK CAPITAL LETTER EPSILON | GREEK SMALL LETTER EPSILON | GREEK LUNATE EPSILON SYMBOL | GREEK REVERSED LUNATE EPSILON SYMBOL | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 917 | U+0395 | 949 | U+03B5 | 1013 | U+03F5 | 1014 | U+03F6 |

UTF-8 | 206 149 | CE 95 | 206 181 | CE B5 | 207 181 | CF B5 | 207 182 | CF B6 |

Numeric character reference | Ε | Ε | ε | ε | ϵ | ϵ | ϶ | ϶ |

Named character reference | Ε | ε | ||||||

DOS Greek | 132 | 84 | 156 | 9C | ||||

DOS Greek-2 | 168 | A8 | 222 | DE | ||||

Windows 1253 | 197 | C5 | 229 | E5 | ||||

TeX | \varepsilon | \epsilon |

- Coptic Eie

Character | Ⲉ | ⲉ | ||
---|---|---|---|---|

Unicode name | COPTIC CAPITAL LETTER EIE | COPTIC SMALL LETTER EIE | ||

Encodings | decimal | hex | decimal | hex |

Unicode | 11400 | U+2C88 | 11401 | U+2C89 |

UTF-8 | 226 178 136 | E2 B2 88 | 226 178 137 | E2 B2 89 |

Numeric character reference | Ⲉ | Ⲉ | ⲉ | ⲉ |

- Latin Open E

Character | Ɛ | ɛ | ᶓ | ᵋ | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | LATIN CAPITAL LETTER OPEN E | LATIN SMALL LETTER OPEN E | LATIN SMALL LETTER OPEN E WITH RETROFLEX HOOK | MODIFIER LETTER SMALL OPEN E | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 400 | U+0190 | 603 | U+025B | 7571 | U+1D93 | 7499 | U+1D4B |

UTF-8 | 198 144 | C6 90 | 201 155 | C9 9B | 225 182 147 | E1 B6 93 | 225 181 139 | E1 B5 8B |

Numeric character reference | Ɛ | Ɛ | ɛ | ɛ | ᶓ | ᶓ | ᵋ | ᵋ |

Character | ɜ | ɝ | ᶔ | ᶟ | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | LATIN SMALL LETTER REVERSED OPEN E | LATIN SMALL LETTER REVERSED OPEN E WITH HOOK | LATIN SMALL LETTER REVERSED OPEN E WITH RETROFLEX HOOK | MODIFIER LETTER SMALL REVERSED OPEN E | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 604 | U+025C | 605 | U+025D | 7572 | U+1D94 | 7583 | U+1D9F |

UTF-8 | 201 156 | C9 9C | 201 157 | C9 9D | 225 182 148 | E1 B6 94 | 225 182 159 | E1 B6 9F |

Numeric character reference | ɜ | ɜ | ɝ | ɝ | ᶔ | ᶔ | ᶟ | ᶟ |

Character | ᴈ | ᵌ | ʚ | ɞ | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | LATIN SMALL LETTER TURNED OPEN E | MODIFIER LETTER SMALL TURNED OPEN E | LATIN SMALL LETTER CLOSED OPEN E | LATIN SMALL LETTER CLOSED REVERSED OPEN E | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 7432 | U+1D08 | 7500 | U+1D4C | 666 | U+029A | 606 | U+025E |

UTF-8 | 225 180 136 | E1 B4 88 | 225 181 140 | E1 B5 8C | 202 154 | CA 9A | 201 158 | C9 9E |

Numeric character reference | ᴈ | ᴈ | ᵌ | ᵌ | ʚ | ʚ | ɞ | ɞ |

- Mathematical Epsilon

Character | 𝚬 | 𝛆 | 𝛦 | 𝜀 | 𝜠 | 𝜺 | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

Unicode name | MATHEMATICAL BOLD CAPITAL EPSILON | MATHEMATICAL BOLD SMALL EPSILON | MATHEMATICAL ITALIC CAPITAL EPSILON | MATHEMATICAL ITALIC SMALL EPSILON | MATHEMATICAL BOLD ITALIC CAPITAL EPSILON | MATHEMATICAL BOLD ITALIC SMALL EPSILON | ||||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 120492 | U+1D6AC | 120518 | U+1D6C6 | 120550 | U+1D6E6 | 120576 | U+1D700 | 120608 | U+1D720 | 120634 | U+1D73A |

UTF-8 | 240 157 154 172 | F0 9D 9A AC | 240 157 155 134 | F0 9D 9B 86 | 240 157 155 166 | F0 9D 9B A6 | 240 157 156 128 | F0 9D 9C 80 | 240 157 156 160 | F0 9D 9C A0 | 240 157 156 186 | F0 9D 9C BA |

UTF-16 | 55349 57004 | D835 DEAC | 55349 57030 | D835 DEC6 | 55349 57062 | D835 DEE6 | 55349 57088 | D835 DF00 | 55349 57120 | D835 DF20 | 55349 57146 | D835 DF3A |

Numeric character reference | 𝚬 | 𝚬 | 𝛆 | 𝛆 | 𝛦 | 𝛦 | 𝜀 | 𝜀 | 𝜠 | 𝜠 | 𝜺 | 𝜺 |

Character | 𝝚 | 𝝴 | 𝞔 | 𝞮 | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | MATHEMATICAL SANS-SERIF BOLD CAPITAL EPSILON | MATHEMATICAL SANS-SERIF BOLD SMALL EPSILON | MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL EPSILON | MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL EPSILON | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 120666 | U+1D75A | 120692 | U+1D774 | 120724 | U+1D794 | 120750 | U+1D7AE |

UTF-8 | 240 157 157 154 | F0 9D 9D 9A | 240 157 157 180 | F0 9D 9D B4 | 240 157 158 148 | F0 9D 9E 94 | 240 157 158 174 | F0 9D 9E AE |

UTF-16 | 55349 57178 | D835 DF5A | 55349 57204 | D835 DF74 | 55349 57236 | D835 DF94 | 55349 57262 | D835 DFAE |

Numeric character reference | 𝝚 | 𝝚 | 𝝴 | 𝝴 | 𝞔 | 𝞔 | 𝞮 | 𝞮 |

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.

Initial epsilon in Lectionary 226, folio 20 verso

- ^
^{a}^{b}^{c}Nick Nicholas: Letters, 2003–2008. (*Greek Unicode Issues*) **^**Colwell, Ernest C. (1969). "A chronology for the letters Ε, Η, Λ, Π in the Byzantine minuscule book hand".*Studies in methodology in textual criticism of the New Testament*. Leiden: Brill. p. 127.**^**Jeffery, Lilian H. (1961).*The local scripts of archaic Greece*. Oxford: Clarendon. pp. 63–64.**^**Jeffery,*Local scripts*, p.24.**^**Jeffery,*Local scripts*, p.114.**^**Jeffery,*Local scripts*, p.138.**^**Jeffery,*Local scripts*, p.89.**^**Thompson, Edward M. (1911).*An introduction to Greek and Latin palaeography*. Oxford: Clarendon. pp. 191–194.

- Hoffman, Paul;
*The Man Who Loved Only Numbers*. Hyperion, 1998. ISBN 0-7868-6362-5.