# How to Compute Power Towers xxx ...

 Infinite Power Tower Calculator x↑↑∞ = ∞x = x x x x ... = ? Enter a number between e-e and e1/e. (between 0.06598803 and 1.44466786) x =

A power tower (also known as a tetration or iterated exponential) is a mathematical operation that extends the idea of exponentiation. The nth power of x, denoted by xn, means x multiplied by itself n times.

The nth iterated exponent of x, usually denoted by

x↑↑n, and less often by nx,

means x exponentiated by itself n times. For instance, x↑↑2 = xx, and x↑↑3 = xxx, etc.

For numbers within a very small range, you can even define an infinite power tower (infinite tetration), in which you exponentiate a number by itself infinitely many times. You can calculate infinite tetration using the calculator on the left. For numbers outside of the given range, the operation does not converge to a single, finite number.

### Growth of Power Towers

Power Towers grow even more rapidly than exponentiated numbers. Consider the first, second, third, and fourth iterated exponents of 2:

12 = 2
22 = 22 = 4
32 = 222 = 24 = 16
42 = 2222 = 216 = 65536

But 52 = 265536 = is a number with 19729 digits. (You can see it here.)

### Infinite Power Towers

The infinite power tower x↑↑∞ (or x) converges so long as

e-e < x < e1/e.

It can be written in closed form using the Lambert W function. The formula is

xxxx ... = -W[-ln(x)]/ln(x)

Some particular values of x↑↑∞ are

1↑↑∞ = 1
2↑↑∞ = 2
(1/e)↑↑∞ = 0.5671432904, the solution to x = -ln(x)
(1/4)↑↑∞ = 1/2

Zero is an example of a number whose power towers 0↑↑n do not converge as n goes to ∞. The value of 0↑↑n oscillates between 0 and 1 depending on whether n is even or odd

10 = 0↑↑1 = 0
20 = 0↑↑2 = 00 = 1
30 = 0↑↑3 = 000 = 01 = 0
40 = 0↑↑4 = 0000 = 00 = 1

(Note that although 00 is an indeterminate form, in power towers it is defined as 1 since the limit of the function f(x) = xx goes to 1 as x goes to 0.)