Phi (capital tiny Φ , φ or ϕ ), in Greek φῖ , is it twenty-and-unième letter of the Greek alphabet.

In Greek old, it was marked /p H /(p followed by a breath) and not /f/.
Puis its value evolved/moved to lead to /f/ the modern pronunciation in Greek.

It is associated with the letter F of the Latin alphabet (but from which the origin is different and comes from the recovery from the old Greek letter Digamma).

Use

The small letter φ or ϕ is used as symbol to note:

• In Mathematical,
• the function φ of Euler, also called Indicating of Euler, which with an entirety positive N associates the cardinal Ensemble of the numbers lower than N and first S with N .
• the Golden section is approximately equal to 1,618034: $\backslash \; varphi\; =\; \backslash \; frac\; \{1\; +\; \backslash \; sqrt\; 5\}\; \{2\}$.
• in Geometry, the Diameter, which is often called “phi”. This symbol is used to indicate the diameter of a circular section. For example ϕ14 means that the diameter is of 14 units of length (Millimètre for example.)
• in spherical Coordonnées, the Angle φ compared to the axis of the coasts ( Z ), also called clean swing angle.
• $\backslash \; Phi$, the function of distribution of a reduced centered normal law
• ϕ is often used like abbreviation for Physique or for Philosophie. But also by many financial institutions because of the comprehensible verbal connotation in the majority of the Latin languages and anglosaxones (phi= FI… nances). Sometimes justified because the applications to the financial field of statistical mathematics are many and increasing.
• the relative Humidity of the air (or degree of hygroscopy), usually noted φ, corresponds to the report/ratio of the pressure partial of steam contained in the air, Pvap, on the steam pressure saturating or vapor tension at the same temperature Psat (T).

Homonyms

• the mark Balsam & Mercier uses also the letter " phi" for their logo defining a perfect balance.
• One finds this letter in the logo of the publisher Phaidon.
• the small pocket of the album “Theology” of Sinéad O' Connor also uses it.