I am curious about how to specify with standard terminology that a certain function is *non-periodic*, in the following sense:

In the simple case of a unary operation $f: X \to X$, this property would specify that, for all $x \in X$:

- $f(x) \ne x$
- $f(f(x)) \ne x$
- $f(f(f(x))) \ne x$

...

For this case, I believe I could simply say that "$f$ has no periodic orbit".

However, the case I am actually curious about is that of a *binary* operation $f: X \times X \to X$. In this case, the property would specify that, for all $a, b \in X$:

- $f(a, b) \notin \{a, b\}$
- $\{f(f(a, b), c), f(c, f(a, b))\} \cap \{a, b\} = \varnothing$
- $\{f(f(f(a, b), c), d), f(f(c, f(a, b)), d), f(d, f(f(a, b), c)), f(d, f(c, f(a, b)))\} \cap \{a, b\} = \varnothing$

...

An example of a binary operation that exhibits this property would be the *addition of positive integers*, i.e., $+: \mathbb{Z}^+ \times \mathbb{Z}^+ \to \mathbb{Z}^+$, since, for all $a, b \in \mathbb{Z}^+$:

- $(a + b) \notin \{a, b\}$
- $(a + b + c) \notin \{a, b\}$
- $(a + b + c + d) \notin \{a, b\}$

...

More generally, I could formally specify this property for any $n$-ary operation $f : X^n \to X$ as follows: $$\forall x \in X, k \in \mathbb{Z}^+ : x \notin F_k(x),$$

where $$F_0(x) = \{x\}$$ $$F_{k+1}(x)=\{f(\mathbf v) : \mathbf v \in X^n \land \exists i:\mathbf v_i \in F_k(x)\}$$

So my question is: does this property have a name? I'd prefer to just call it by its name, if there exists a standard term for it, rather than formally specifying it (e.g., let $f$ be a *[fill-in-the-blank]* binary operator on $X$).