Iterated Binary Operation - Definition

Definition

Denote by aj,k, with j ≥ 0 and k ≥ j, the finite sequence of length k − j of elements of S, with members (ai), for j ≤ i < k. Note that if k = j, the sequence is empty.

For f : S × S, define a new function Fl on finite nonempty sequences of elements of S, where

F_l(\mathbf{a}_{0,k})= \begin{cases} a_0, &k=1\\ f(F_l(\mathbf{a}_{0,k-1}), a_k), &k>1 \end{cases}.

Similarly, define

F_r(\mathbf{a}_{0,k})= \begin{cases} a_0, &k=1\\ f(a_0, F_r(\mathbf{a}_{1,k})), &k>1 \end{cases}.

If f has a unique left identity e, the definition of Fl can be modified to operate on empty sequences by defining the value of Fl on an empty sequence to be e (the previous base case on sequences of length 1 becomes redundant). Similarly, Fr can be modified to operate on empty sequences if f has a unique right identity.

If f is associative, then Fl equals Fr, and we can simply write F. Moreover, if an identity element e exists, then it is unique (see Monoid).

If f is commutative and associative, then F can operate on any non-empty finite multiset by applying it to an arbitrary enumeration of the multiset. If f moreover has an identity element e, then this is defined to be the value of F on an empty multiset. If f is idempotent, then the above definitions can be extended to finite sets.

If S also is equipped with a metric or more generally with topology that is Hausdorff, so that the concept of a limit of a sequence is defined in S, then an infinite iteration on a countable sequence in S is defined exactly when the corresponding sequence of finite iterations converges. Thus, e.g., if a0, a1, a2, a3, ... is an infinite sequence of real numbers, then the infinite product is defined, and equal to if and only if that limit exists.

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