# Notation for repeated application of function

If I have the function $f(x)$ and I want to apply it $n$ times, what is the notation to use?

For example, would $f(f(x))$ be $f_2(x)$, $f^2(x)$, or anything less cumbersome than $f(f(x))$? This is important especially since I am trying to couple this with a limit toward infinity.

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You could define the notation recursively as a sequence of functions.

Let $f_{n+1}(x) = f(f_n(x))$ for $n \geq 2$ with $f_1(x) = f(x)$.

Sequence notation of this type is so generic that the reader will be forced to consult your definition, which will avoid any possible misinterpretation.

In the course I took on bifurcation theory we used the notation $$f^{\circ n}(x).$$

You can use the notation $f^n$ to denote the composition of the function with itself $n$ times, though this may also mean the product of $f$ with itself $n$ times. Just make sure you define your notation at the start.

You can use $f^n(x)$ BUT be sure to tell the reader that you mean functional iteration, not $(f(x))^n$.

If you take function iteration as a fold of self-composition, you can use a sum-like notation:

$\bigcirc^nf = \underbrace{f \circ \dots \circ f}_{n\:\text{times}}$

Where:

$\left({\bigcirc^0f}\right)(x) = x$

Granted, this is not very compact, and I would prefer to typeset the limit directly above the circle. That aside, it does combine tolerably with limit notation:

$\lim_{n\rightarrow\infty}\bigcirc^nf$

There are two common notations for this, in two different communities:

1. People in quantum physics, functional analysis and similar topics write everything without parentheses and for them, naturally $B^2x=BBx=B(B(x))$.

2. Some people in albegra treat functions as “modifiers” and when $\alpha$ is a function, then they write the image of $x$ by $\alpha$ as $x\alpha$. For them, $x\alpha^2$ is what I would write $\alpha(\alpha(x))$.

At any case, as others mention, you can use whatever you like, as long as you clarify your notations at the beginning. If you need it a lot, I would go for one of $$f^n(x),\qquad f_n(x),\qquad f^{\circ n}(x),\qquad f^{[n]}(x).$$

You may also use Lagrange’s notation of derivative $\ f^{(n)}(x)\$ instead of more commonly used notations $\ f^n(x)\$ or $\ f_n(x)\$.

EDIT:
Or you can use left indices: $\ ^n f(x)\$ or $\ _n f(x)\$ or $\ ^{(n)} f(x)\$ or $\ _{(n)} f(x)\$.

There is a notation proposal for iteration of functions. Check if the following can be useful for your purposes: Salov, Valerii. “Notation for Iteration of Functions, Iteral”, arXiv:1207.0152, June 30, 2012, https://arxiv.org/abs/1207.0152; Salov, Valerii. “Inevitable Dottie Number. Iterals of cosine and sine”, arXiv:1212.1027, December 1, 2012, https://arxiv.org/abs/1212.1027.