Epsilon (uppercase/lowercase Ε ε), is the letter of the Greek alphabet, used to represent the "e" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 5. Letters that came from it include the Roman E and Cyrillic Е. It is also used in calculus to represent a very small number (particularly in proofs involving limit). In set theory, a typographical variant of epsilon can be used to represent the statement "a is a member of set A" (symbolically formula_1).