Eta (uppercase/lowercase Η η) is a letter of the Greek alphabet. In very early Greek writing it stood for the "h" sound, but in Classical Greek it stood for a long "e" sound. In Modern Greek, it is pronounced as "i". In the system of Greek numerals, it has a value of 8. Letters that came from it include the Roman H and Cyrillic И.
In mathematics, the lowercase η is used to represent Dirichlet eta function.[1] The same letter is also used to represent conformal time in cosmology, efficiency in telecommunications and elasticity in economics.[2]
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