Rho

Look up Ρ or ρ 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		Qoppa
	San		Sampi
Other characters
	Stigma		Sho
	Heta
Greek diacritics

Rho (uppercase Ρ, lowercase ρ or ϱ) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Semitic Rêš "head" (see Resh). Its uppercase form is not to be confused with the Roman letter P, however both are typeset using the same glyph.

The letter Rho has religious significance within Christianity. When fused within a single typespace with the Greek letter Chi, it forms the labarum, which is used to represent Jesus Christ.

Rho is generally classed as a liquid (together with lambda and sometimes the nasals mu and nu), which has important implications for morphology. It is pronounced similarly to the letter r in languages with a Latin-derived alphabet. In polytonic orthography a rho at the beginning of a word is conventionally written with a rough breathing (equivalent to h) — ῥ — and a double rho within a word is written with a smooth breathing over the first rho and a rough breathing over the second — ῤῥ — apparently reflecting an aspirated or voiceless pronunciation in Ancient Greek, hence the various Greek-derived English words which start with rh or contain rrh. The name of the letter is written in Greek as ῥῶ (polytonic) or ρω/ρο (monotonic).

Letters that arose from rho include Roman R and Cyrillic Er (Р).

If a person is a former Rhodes Scholar, they are entitled to use the Greek letter rho as a designation of their status. When used, the symbol should precede the name. ^[1]

Common notational uses

The characters Ρ, ρ and ϱ are also used outside its Greek alphabetical context in science and mathematics.

• In physics to represent:
  • Density (ρ)
  • Resistivity (ρ)
  • Charge density (ρ)

• In mathematics to represent:
  • The radius in a system of spherical polar coordinates
  • The correlation coefficient of a population parameter
  • The spectral radius of a matrix A denoted as ρ(A)
  • The plastic number

• In economics to represent the discount rate of future pence cash flows

• In molecular biology to represent the Rho protein responsible for termination of RNA synthesis. In such occasions, it is often represented as ϱ ("rho symbol" U+03F1), to avoid confusion with the Latin letter p

• In ecology to represent the population damping ratio where ρ = λ1 / |λ2|.

• In programming
  • For LaTeX, the symbols are P (Ρ), \rho (ρ), and \varrho ()
  • The lower-case rho ρ has a special meaning in the APL programming language

• In statistics to represent Spearman's rank correlation coefficient, commonly known as Spearman's rho

• In Options theory to represent the rate of change of a portfolio with respect to interest rates

Computing

In Unicode, the upper and lower case are encoded at U+03A1 and U+03C1 respectively.

See also

• Р, р - Er (Cyrillic)
• R, r - Latin
• P, p - Latin

References

1. ^ "Oxford University Calendar: Notes on Style". Oxford University Gazette. 2009-03-26. http://www.ox.ac.uk/gazette/calendar/style.pdf. Retrieved 2009-06-02.