Delta delta.

-Weierstrass, the father of modern analysis, put forward the famous definition of function limit "ε-δ" on the basis of Cauchy and Abel's work, which is the strict definition of the limit we are studying now. Through the concept of limit, we also realize an important fact in mathematical analysis: many numbers can only be defined by limit.

Interesting explanation

The definition of "ε-δ" of function limit is just like the Monkey King ε and Tathagata δ.

No matter how hard the Monkey King ε tries, he can't escape the palm of Tathagata δ. As long as | x-x0 | < δ, | f (x)-a | < ε. Tathagata δ can make the Monkey King ε arbitrarily smaller by controlling its own size.

The above is from the net text of the literary mathematics gentleman. mathpretty.com/837 1.html