### Rho (letter)

Look up or Ρ in , the free dictionary.ρ |

**Rho** (glyph.

## Contents

## Uses

### Greek

Rho is classed as a liquid consonant (together with lambda and sometimes the nasals mu and nu), which has important implications for morphology. In both Ancient and Modern Greek, it represents a trilled or tapped r.

In polytonic orthography a rho at the beginning of a word is written with a rough breathing (equivalent to *h*) — ῥ *rh* — 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 *rrh* — 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).

### Other alphabets

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

### Mathematics and science

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

- In the physical sciences to represent:
- Densities: mass density, or charge density (
*ρ*) - Resistivity (
*ρ*) - Rho meson (ρ
^{+}, ρ^{-}, ρ^{0}) - General quantum states
- Hammett Equation, ρ is used to represent the reaction constant, this is independent of the position and nature of the substituents of the benzene ring.

- Densities: mass density, or charge density (

- In mathematics to represent:
- The radius in the cylindrical or spherical coordinate systems
- The correlation coefficient of a population parameter
- The spectral radius of a matrix $A$ denoted as $\backslash rho(A)$
- The plastic number
- The sensitivity to interest rates of a pricing function
- The expected return of a given policy ($\backslash pi$) in reinforcement learning, denoted $\backslash rho^\backslash pi$

- 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 theology, it is used to sometimes denote the crucifixion of Jesus.**
**

- In computer programming
- For LaTeX, the symbols are
`\rho`

($\backslash rho$), and`\varrho`

($\backslash varrho$) (note:`\Rho`

is not defined by default) - The lower-case rho "ρ" has a special meaning in the APL programming language

- For LaTeX, the symbols are

- 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

### Chi Rho

The letter rho overlaid with chi forms the *Chi Rho* symbol, used to represent Jesus Christ.

### Rhodes Scholars

Former Rhodes Scholars are entitled to use the Greek letter rho as a designation of their status. When used, the symbol should precede the name.^{[1]}

## Character encodings

- Greek Rho
^{[2]}

- Greek Rho symbols

- Coptic Ro

- Technical Symbol

- Mathematical Rho

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.