This letter denotes ordinal numbers that are the fixed point of the exponential map. The Galois group of a field extension is linked to the properties of elliptic curves in a conjecture denoted by this letter that was proved by Ken Ribet. Transitions denoted by this letter can be used to simplify the state diagram of a nondeterministic (15[1])finite (15[1])automaton. For any positive number denoted by this letter, there exists a positive (*) delta that a function is arbitrarily closed to L in a definition of the limit. The identity element of the concatenation operation is denoted by this letter. A quantity denoted that characterizes the shapes of conic sections is called eccentricity. For 10 points, identify this Greek letter that denotes the empty strings, which can also denote an arbitrarily small number. ■END■