TheoremBase

Partial Iterates of a Two-Variable Map, Row by Row and Column by Column

For a map of two variables into a set with a binary operation, the partial iterates along the natural-number variable, taken row by row or column by column, are the values of exactly one map of two variables, and swapping the two arguments of a map gives exactly one map.

Statement

In the setting of Routine Facts Used Without Citation: Arithmetic, Maps, Predicativity and Indices, let ∗\ast be a binary operation on a set XX, let AA and BB be sets, and for N∈NN\in\mathbb{N} let [N][N] be as in Intervals of Natural Numbers (ii). For a map ff on a Cartesian product, f(x,y)f(x,y) stands for f((x,y))f((x,y)), as in Sets and Maps: Ordinary Notation (v). For N∈NN\in\mathbb{N} and an expression t(n)t(n) in XX for n∈[N]n\in[N], ∗n=1Nt(n)\mathop{\ast}\limits_{n=1}^{N}t(n) is the iterated operation of n↦t(n)n\mapsto t(n) on [N][N].

For every map f:A×N→Xf:A\times\mathbb{N}\to X there is exactly one map g:N×A→Xg:\mathbb{N}\times A\to X with g(N,x)=∗n=1Nf(x,n)g(N,x)=\mathop{\ast}\limits_{n=1}^{N}f(x,n) for all N∈NN\in\mathbb{N} and x∈Ax\in A.

For every map f:N×A→Xf:\mathbb{N}\times A\to X there is exactly one map g:N×A→Xg:\mathbb{N}\times A\to X with g(N,y)=∗m=1Nf(m,y)g(N,y)=\mathop{\ast}\limits_{m=1}^{N}f(m,y) for all N∈NN\in\mathbb{N} and y∈Ay\in A.

For every map f:A×B→Xf:A\times B\to X there is exactly one map h:B×A→Xh:B\times A\to X with h(y,x)=f(x,y)h(y,x)=f(x,y) for all x∈Ax\in A and y∈By\in B.

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