For 10 points, name this Greek letter often paired in a certain kind of limit proof with delta, which generally denotes any arbitrarily small number.
Herbrand's theorem was first proven using a "calculus" denoted by this letter, expanded upon by Ackermann and developed by Hilbert and Bernays.  The best-known of the lozenge, or rhombus, algorithms is named after Wynn and this letter.  Serre's conjecture about (*) Frey's elliptical curve was solved by Ribet and known by this letter; once solved, Fermat's Last Theorem could be directly implied by the Taniyama-Shimura Conjecture.  Another method denoted by this letter was developed by Weierstrass in his work on infinite sequences; that method was extended to its modern form by Cauchy and is important in limits.  For 10 points, name this Greek letter often paired in a certain kind of limit proof with delta, which generally denotes any arbitrarily small number.
Herbrand's theorem was first proven using a "calculus" denoted by this letter, expanded upon by Ackermann and developed by Hilbert and Bernays.  
{epsilon} (2) [DW]