# Contents

## Definition

### Algebra of function on a set

For $R$ a ring and $S$ a set, the set of functions $S \to R$ (to the underlying set of $R$) is itself naturally an associative algebra over $R$, where addition and multiplication is given pointwise in $S$ by addition and multiplication in $R$: for $f_1, f_2 \colon S \to R$ their sum is the function

$(f_1 + f_2) \colon s \mapsto f_1(s) + f_2(s) \in R \,,$

their product is the function

$(f_1 \cdot f_2) \colon s \mapsto f_1(s) f_2(s) \in R$

and the ring inclusion $R \to [S,R]$ is given by sending $r \in R$ to the constant function with value $r$.

### Algebra of functions on an $\infty$-stack

More generally, in the context of (∞,1)-topos theory and higher algebra, there is a notion of function algebras on ∞-stacks?.

## Properties

### Relation to free modules

If $S$ is a finite set or else if one restricts to functions that are non-vanishing only for finitely many elements in $S$, then the algebra of functions with values in $R$ also forms the free module over $R$ generated by $S$.

### Duality between algebra and geometry

Sending spaces to their suitable algebras of functions constitutes a basic duality operation that relates geometry and algebra. For more on this see at Isbell duality.

Last revised on November 22, 2016 at 19:57:59. See the history of this page for a list of all contributions to it.