Phi (letter)

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For other uses, see Phi.

Look up Φ, φ in Wiktionary, the free dictionary.

Greek alphabet

Αα Alpha	Νν Nu
Ββ Beta	Ξξ Xi
Γγ Gamma	Οο Omicron
Δδ Delta	Ππ Pi
Εε Epsilon	Ρρ Rho
Ζζ Zeta	Σσς Sigma
Ηη Eta	Ττ Tau
Θθ Theta	Υυ Upsilon
Ιι Iota	Φφ Phi
Κκ Kappa	Χχ Chi
Λλ Lambda	Ψψ Psi
Μμ Mu	Ωω Omega
Obsolete letters
Digamma	Image:Qoppa uc lc.svg Qoppa
Image:San uc lc.svg San	Image:Sampi uc lc.svg Sampi

Greek diacritics

Phi (uppercase Φ, lowercase φ or ϕ), pronounced [fi] in modern Greek and as [faɪ] in English, is the 21st letter of the Greek alphabet. In modern Greek, it represents [f], a voiceless labiodental fricative. In Ancient Greek it represented [pʰ], an aspirated voiceless bilabial plosive (from which English ultimately inherits the spelling "ph" in words derived from Greek). In the system of Greek numerals it has a value of 500 (φʹ) or 500,000 (͵φ).

The lower-case letter Failed to parse (Missing texvc executable; please see math/README to configure.): \varphi \,

(or often its variant, Failed to parse (Missing texvc executable; please see math/README to configure.): \phi \,

) is used as a symbol for:

The golden ratio 1.618... in mathematics, art, and architecture.
Euler's totient function φ(n) in number theory; also called Euler's phi function.
The probability density function of the normal distribution in mathematics and statistics.
An angle, typically the second angle mentioned, after θ (theta). Especially:
- The argument of a complex number.
- The phase of a wave in signal processing.
- In spherical coordinates, mathematicians usually refer to phi as the polar angle (from the z-axis). The convention in physics is to use phi as the azimuthal angle (from the x-axis).
- One of the dihedral angles in the backbones of proteins.
- Internal or effective angle of friction
Any function in mathematics.
Electric potential in physics.
The work function in electronics.
A shorthand representation for an aromatic functional group in organic chemistry
The fugacity coefficient in thermodynamics
The ratio of free energy destabilizations of protein mutants in phi value analysis
A sentence in first-order logic.

The upper-case letter Φ is used as a symbol for:

The magnetic flux and electric flux in physics, with subscripts distinguishing the two.
The cumulative distribution function of the normal distribution in mathematics and statistics.
Philosophy.
Strength (or resistance) reduction factor in structural engineering, used to account for statistical variabilities in materials and construction methods.
The number of phases in a power system in electrical engineering, for example 1Φ for single phase, 3Φ for three phase.
The symbol of the voiceless bilabial fricative in the international phonetic alphabet.

The diameter symbol in engineering, ⌀, is often incorrectly referred to as "phi". This symbol is used to indicate the diameter of a circular section, for example ⌀14 means the diameter of the circle is 14 units.

Computing

In Unicode, there are multiple forms of the phi letter:

upper case:
- U+03A6 GREEK CAPITAL LETTER PHI (Φ): Greek capital letter phi
lower case:
- U+03C6 GREEK SMALL LETTER PHI (φ): letter phi, used in Greek texts.
- U+03D5 GREEK PHI SYMBOL (Failed to parse (Missing texvc executable; please see math/README to configure.): \phi\,\!

): phi symbol, for mathematical and technical contexts. ^[1]

In HTML/XHTML, the upper and lower case phi character entity references are Φ (Φ) and φ (φ) respectively.

In LaTeX, the math symbols are \Phi (Failed to parse (Missing texvc executable; please see math/README to configure.): \Phi\,\! ), \phi (Failed to parse (Missing texvc executable; please see math/README to configure.): \phi \,\! ), and \varphi (Failed to parse (Missing texvc executable; please see math/README to configure.): \varphi\,\! ).

In some browsers (e.g. Internet Explorer 6), the shapes of the U+03C6 GREEK SMALL LETTER PHI (which should be viewed as a curl) and U+03D5 GREEK PHI SYMBOL (which should be viewed as a circle crossed by a slash) are exchanged. Compare this samples to check your browser:

Phi letter: right Failed to parse (Missing texvc executable; please see math/README to configure.): \varphi\,\!
possible wrong φ: Phi symbol: right Failed to parse (Missing texvc executable; please see math/README to configure.): \phi\,\!
possible wrong ϕ