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Mu (uppercase Μ, lowercase μ; Ancient Greek μῦ [mŷː], Greek: μι or μυ—both [mi]) or my^{[1]} is the 12th letter of the Greek alphabet. In the system of Greek numerals it has a value of 40.^{[2]} Mu was derived from the Egyptian hieroglyphic symbol for water, which had been simplified by the Phoenicians and named after their word for water, to become 𐤌^{img} (mem). Letters that derive from mu include the Roman M and the Cyrillic М. It is also used as coefficient of friction.
In Ancient Greek, the name of the letter was written μῦ and pronounced [mŷː].
In Modern Greek, the letter is spelled μι and pronounced [mi]. In polytonic orthography, it is written with an acute accent: μί.^{[3]}^{[4]}
The lowercase letter mu (μ) is used as a special symbol in many academic fields. Uppercase mu is not used, because it appears identical to Latin M.
"μ" is conventionally used to denote certain things; however, any Greek letter or other symbol may be used freely as a variable name.
In classical physics and engineering:
In particle physics:
In thermodynamics:
In type theory:
In chemistry:
In biology:
In pharmacology:
The Olympus Corporation manufactures a series of digital cameras called Olympus µ [mju:]^{[9]} (known as Olympus Stylus in North America)
In phonology, it often stands for mora. In syntax, μP (mu phrase) can be used as the name for a functional projection.^{[10]}. Celtic specialists sometimes use /µ/ to represent an Old Irish nasalized labial fricative of uncertain articulation, the ancestor of the sound represented by Modern Irish mh.
Character  Μ  μ  µ  Ⲙ  ⲙ  

Unicode name  GREEK CAPITAL LETTER MU  GREEK SMALL LETTER MU  MICRO SIGN  COPTIC CAPITAL LETTER MI  COPTIC SMALL LETTER MI  
Encodings  decimal  hex  decimal  hex  decimal  hex  decimal  hex  decimal  hex 
Unicode  924  U+039C  956  U+03BC  181  U+00B5  11416  U+2C98  11417  U+2C99 
UTF8  206 156  CE 9C  206 188  CE BC  194 181  C2 B5  226 178 152  E2 B2 98  226 178 153  E2 B2 99 
Numeric character reference  Μ  Μ  μ  μ  µ  µ  Ⲙ  Ⲙ  ⲙ  ⲙ 
Named character reference  Μ  μ  µ  
ISO/IEC 88591  181  B5  
ISO/IEC 88597  204  CC  236  EC  
Code page 437, 850  230  E6  230  E6  
Code page 737  139  8B  163  A3  
Code page 851, 869  183  B7  230  E6  
Code page 1253  204  CC  236  EC  
Roman8, Roman9  243  F3  243  F3  
TeX  \mu  \micro 
Character  𝚳  𝛍  𝛭  𝜇  𝜧  𝝁  

Unicode name  MATHEMATICAL BOLD CAPITAL MU 
MATHEMATICAL BOLD SMALL MU 
MATHEMATICAL ITALIC CAPITAL MU 
MATHEMATICAL ITALIC SMALL MU 
MATHEMATICAL BOLD ITALIC CAPITAL MU 
MATHEMATICAL BOLD ITALIC SMALL MU  
Encodings  decimal  hex  decimal  hex  decimal  hex  decimal  hex  decimal  hex  decimal  hex 
Unicode  120499  U+1D6B3  120525  U+1D6CD  120557  U+1D6ED  120583  U+1D707  120615  U+1D727  120641  U+1D741 
UTF8  240 157 154 179  F0 9D 9A B3  240 157 155 141  F0 9D 9B 8D  240 157 155 173  F0 9D 9B AD  240 157 156 135  F0 9D 9C 87  240 157 156 167  F0 9D 9C A7  240 157 157 129  F0 9D 9D 81 
UTF16  55349 57011  D835 DEB3  55349 57037  D835 DECD  55349 57069  D835 DEED  55349 57095  D835 DF07  55349 57127  D835 DF27  55349 57153  D835 DF41 
Numeric character reference  𝚳  𝚳  𝛍  𝛍  𝛭  𝛭  𝜇  𝜇  𝜧  𝜧  𝝁  𝝁 
Character  𝝡  𝝻  𝞛  𝞵  

Unicode name  MATHEMATICAL SANSSERIF BOLD CAPITAL MU 
MATHEMATICAL SANSSERIF BOLD SMALL MU 
MATHEMATICAL SANSSERIF BOLD ITALIC CAPITAL MU 
MATHEMATICAL SANSSERIF BOLD ITALIC SMALL MU  
Encodings  decimal  hex  decimal  hex  decimal  hex  decimal  hex 
Unicode  120673  U+1D761  120699  U+1D77B  120731  U+1D79B  120757  U+1D7B5 
UTF8  240 157 157 161  F0 9D 9D A1  240 157 157 187  F0 9D 9D BB  240 157 158 155  F0 9D 9E 9B  240 157 158 181  F0 9D 9E B5 
UTF16  55349 57185  D835 DF61  55349 57211  D835 DF7B  55349 57243  D835 DF9B  55349 57269  D835 DFB5 
Numeric character reference  𝝡  𝝡  𝝻  𝝻  𝞛  𝞛  𝞵  𝞵 
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.
Look up Μ or μ in Wiktionary, the free dictionary. 
Amplification factor or voltage gain is the amount the signal at the control grid is increased in amplitude after passing through the tube, which is also referred to as the Greek letter μ (mu) or voltage gain (V_{g}) of the tube.