Tetration is a binary mathematical operator defined by the recurrence relation:  More intuitively, with b copies of a. is pronounced "a tetrated to b" or "to-the-b a."

Tetration leads to very large numbers, even with small inputs. For example, , which has 3638334640025 digits.

## Generalizing

The problem with the above definition is that works only for nonnegative integers b. What is , for example?

Generalizing b to the real numbers is a tricky and interesting problem. We present a solution proposed by Daniel Geisler of http://tetration.org/. Heavy differential calculus is ahead, so be warned. This is intended to be a gentle introduction; visit the original page if you don't need a tutorial.

Tetration is part of a class of general problems involving function iteration. Function iteration is defined by the recurrence relation , , so , , etc. Tetration could be defined as where . So if we can define for real , we're all set for defining continuous tetration.

First, we translate f so that . This simplifies a lot of the math. Then we consider the Maclaurin series (Taylor series around 0) of : This converges to for for some radius . What we need to do is find .

### Constant term

First, because .

### First derivative

To compute , we need to find . Define for convenience: (Chain Rule)   Plugging in , we can take advantage of the fact that : Since we'll see them a lot, we'll define , , etc. So we can write our latest finding as .

### Second derivative

This one is somewhat nastier. Again define : (Chain Rule) (Product Rule) (Product Rule) Setting :    ### Third derivative (Chain Rule + Product Rule)  (Product Rule, 2x) (combining like terms)