Definition:Bourbaki Assembly
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Definition
The notion of assembly is an example of a collation used in 1968: Nicolas Bourbaki: Theory of Sets.
An assembly is a succession of signs written one after another.
Certain signs which are not variables are allowed to be joined in pairs by links, as follows:
- $\overbrace {\tau A \Box}^{} A'$
Link
In an assembly, certain signs which are not variables can be joined in (ordered) pairs by links, as follows:
- $\overbrace {\tau A \Box}^{} A'$
where the $\overbrace {}{}$ depicts a link between $\tau$ and $\Box$.
Construction of Assemblies
Concatenation
Let $\mathbf A$ and $\mathbf B$ denote assemblies.
Then $\mathbf {A B}$ denotes the assembly formed from writing the assembly $\mathbf B$ immediately to the right of assembly $\mathbf A$.
Extracting the Variable
Let $\mathbf A$ denote an assembly and let $x$ be a letter.
Then the assembly $\map {\tau_x} {\mathbf A}$ is constructed as follows:
- $(1): \quad$ Form the assembly $\tau \mathbf A$
- $(2): \quad$ Link each occurrence of $x$ wherever it appears in $\mathbf A$ to the $\tau$ written to the left of $\mathbf A$
- $(3): \quad$ Replace $x$ by $\Box$ wherever it occurs in $\mathbf A$.
The assembly denoted $\map {\tau_x} {\mathbf A}$ therefore does not contain $x$.
Replacement
Let $\mathbf A$ and $\mathbf B$ denote assemblies.
We denote by:
- $\paren {\mathbf B \mid x} \mathbf A$
the assembly obtained by replacing $x$, wherever it occurs, by $\mathbf B$.
This is read::
- $\mathbf B$ replaces $x$ in $\mathbf A$.
If $x$ does not appear in $\mathbf A$, then $\paren {\mathbf B \mid x} \mathbf A$ is identical with $\mathbf A$.
In particular:
- $\paren {\mathbf B \mid x} \map {\tau_x} {\mathbf A}$
is identical with $\map {\tau_x} {\mathbf A}$.
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Notation
The specific symbology used by Bourbaki in Theory of Sets has not been rendered accurately here, as the author of this page has not been able to establish a method by which to do it.
However, the intent has been expressed as accurately as possible.
Also see
- Results about Bourbaki assemblies can be found here.
Sources
- 1968: Nicolas Bourbaki: Theory of Sets ... (previous) ... (next): Chapter $\text I$: Description of Formal Mathematics: $1$. Terms and Relations: $1$. Signs and Assemblies