**Epsilon** (UK: /ɛpˈsaɪlən/,^{[1]} US: /ˈɛpsɪlɒn/; uppercase **Ε**, lowercase **ε** or lunate **ϵ**; Greek: έψιλον) is the fifth letter of the Greek alphabet, corresponding phonetically to a mid front unrounded vowel /e/. In the system of Greek numerals it also 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.

The uppercase form of epsilon looks identical to Latin E but has its own code point in Unicode: U+0395 Ε GREEK CAPITAL LETTER EPSILON. 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 number "3" and is encoded U+03B5 ε GREEK SMALL LETTER EPSILON. The other, also known as lunate or uncial epsilon and inherited from earlier uncial writing,^{[2]}^{[3]} looks like a semicircle crossed by a horizontal bar: it is encoded U+03F5 ϵ GREEK LUNATE EPSILON SYMBOL. 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.^{[2]} In TeX, `\epsilon`

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

( ) denotes the reversed-3 form.

There is also a 'Latin epsilon', ɛ or "open e", which looks similar to the Greek lowercase epsilon. It is encoded in Unicode as U+025B ɛ LATIN SMALL LETTER OPEN E and U+0190 Ɛ LATIN CAPITAL LETTER OPEN E and is used as an IPA phonetic symbol. The lunate or uncial epsilon provided inspiration for the euro sign, €.^{[4]}

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 Σ (sigma). The symbol , first used in set theory and logic by Giuseppe Peano and now used in mathematics in general for set membership ("belongs to") evolved from the letter epsilon, since the symbol was originally used as an abbreviation for the Latin word "**e**st". In addition, mathematicians often read the symbol ∈ as "element of", as in "1 is an element of the natural numbers" for , for example. As late as 1960, ε itself was used for set membership, while its negation "does not belong to" (now ∉) was denoted by ε' (epsilon prime).^{[5]} Only gradually did a fully separate, stylized symbol take the place of epsilon in this role. In a related context, Peano also introduced the use of a backwards epsilon, ϶, for the phrase "such that", although the abbreviation "s.t." is occasionally used in place of ϶ in informal cardinals.

## History

### Origin

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 as 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.^{[6]}

### Sound value

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.^{[7]} 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 BC, 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.

### Epichoric alphabets

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/.^{[8]} 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 .^{[9]}

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.^{[10]}^{[11]} This tack glyph was used elsewhere also as a form of "Heta", i.e. for the sound /h/.

### Glyph variants

After the establishment of the canonical classical Ionian (Eucleidean) Greek alphabet, 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.^{[12]} 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 |
---|---|---|---|---|

## Uses

### International Phonetic Alphabet

Despite its pronunciation as mid, in the International Phonetic Alphabet, the Latin epsilon /ɛ/ represents open-mid front unrounded vowel, as in the English word *pet* /pɛt/.

### Symbol

The uppercase Epsilon is not commonly used outside of the Greek language because of its similarity to the Latin letter E. However, it is commonly used in structural mechanics with Young's Modulus equations for calculating tensile, compressive and areal strain.

The Greek lowercase epsilon `ε`

, the lunate epsilon symbol `ϵ`

, or the Latin lowercase epsilon `ɛ`

(see above) is used in a variety of places:

- In engineering mechanics, strain calculations ϵ = increase of length / original length. Usually this relates to extensometer testing of metallic materials.
- In mathematics
- (particularly calculus), an arbitrarily small positive quantity is commonly denoted ε; see (ε, δ)-definition of limit.
- In reference to this, the late mathematician Paul Erdős also used the term "epsilons" to refer to children (Hoffman 1998, p. 4).

- Hilbert introduced epsilon terms as an extension to first order logic; see epsilon calculus.
- it is used to represent the Levi-Civita symbol.
- it is used to represent dual numbers:
*a*+*bε*, with*ε*^{2}= 0 and*ε*≠ 0. - it is sometimes used to denote the Heaviside step function.
^{[13]} - 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}, ...}.

- (particularly calculus), an arbitrarily small positive quantity is commonly denoted ε; see (ε, δ)-definition of limit.
- In computer science, it often represents 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 computer science, the machine epsilon indicates the upper bound on the relative error due to rounding in floating point arithmetic.
- In physics,
- it indicates the permittivity of a medium; with the subscript 0 (ε
_{0}) it is the permittivity of free space. - it can also indicate the strain of a material (a ratio of extensions).

- it indicates the permittivity of a medium; with the subscript 0 (ε
- In automata theory, it shows a transition that involves no shifting of an input symbol.
- In astronomy,
- it stands for the fifth-brightest star in a constellation (see Bayer designation).
- Epsilon is the name for the most distant and most visible ring of Uranus.
- In planetary science, ε denotes the axial tilt.

- In chemistry, it represents the molar extinction coefficient of a chromophore.
- In economics, ε refers to elasticity.
- In statistics,
- it is used to refer to error terms.
- it also can to refer to the degree of sphericity in repeated measures ANOVAs.

- In agronomy, it is used to represent the "photosynthetic efficiency" of a particular plant or crop.

## Unicode

- Greek Epsilon

Preview | 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, ε | ϵ, 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

Preview | 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

Preview | 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 |

Preview | 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 |

Preview | 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

Preview | 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 |

Preview | 𝛜 | 𝜖 | 𝝐 | |||
---|---|---|---|---|---|---|

Unicode name |
MATHEMATICAL BOLD EPSILON SYMBOL |
MATHEMATICAL ITALIC EPSILON SYMBOL |
MATHEMATICAL BOLD ITALIC EPSILON SYMBOL | |||

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

Unicode | 120540 | U+1D6DC | 120598 | U+1D716 | 120656 | U+1D750 |

UTF-8 | 240 157 155 156 | F0 9D 9B 9C | 240 157 156 150 | F0 9D 9C 96 | 240 157 157 144 | F0 9D 9D 90 |

UTF-16 | 55349 57052 | D835 DEDC | 55349 57110 | D835 DF16 | 55349 57168 | D835 DF50 |

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

Preview | 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 |

Preview | 𝞊 | 𝟄 | ||
---|---|---|---|---|

Unicode name |
MATHEMATICAL SANS-SERIF BOLD EPSILON SYMBOL |
MATHEMATICAL SANS-SERIF BOLD ITALIC EPSILON SYMBOL | ||

Encodings | decimal | hex | decimal | hex |

Unicode | 120714 | U+1D78A | 120772 | U+1D7C4 |

UTF-8 | 240 157 158 138 | F0 9D 9E 8A | 240 157 159 132 | F0 9D 9F 84 |

UTF-16 | 55349 57226 | D835 DF8A | 55349 57284 | D835 DFC4 |

Numeric character reference | 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

Initial epsilon in Lectionary 226, folio 20 verso

## References

**^**"epsilon".*Oxford English Dictionary*(Online ed.). Oxford University Press. (Subscription or participating institution membership required.)- ^
^{a}^{b}Nick Nicholas: Letters Archived 2012-12-15 at Archive.today, 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.**^**"European Commission – Economic and Financial Affairs – How to use the euro name and symbol". Ec.europa.eu. Retrieved 7 April 2010.Inspiration for the € symbol itself came from the Greek epsilon, ϵ – a reference to the cradle of European civilization – and the first letter of the word Europe, crossed by two parallel lines to ‘certify’ the stability of the euro.

**^**Halmos, Paul R. (1960).*Naive Set Theory*. New York: Van Nostrand. pp. 5–6. ISBN 978-1614271314.**^**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.**^**Nicholas, Nick (2005). "Proposal to add Greek epigraphical letters to the UCS" (PDF). Archived from the original (PDF) on 2006-05-05. Retrieved 2010-08-12.**^**Jeffery,*Local scripts*, p.89.**^**Thompson, Edward M. (1911).*An introduction to Greek and Latin palaeography*. Oxford: Clarendon. pp. 191–194.**^**Weisstein, Eric W. "Delta Function".*mathworld.wolfram.com*. Retrieved 2019-02-19.

## Further reading

Look up or Ε in Wiktionary, the free dictionary.ɛ |

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