# Ring based on families of integers - The Function Ring

• Mar 4th 2013, 07:16 PM
Bernhard
Ring based on families of integers - The Function Ring
I am trying to get a full understanding of Cohn Example 4 on page 9 - see attachment.

A formal and rigorous example showing clearly the nature and role of the families (and their indicies) involved would help enormously.

Cohn, page 9, Example 4 states:

"Let I be any set and denote by $Z^I$ the set of all functions from I to Z.

Thus the elements of $Z^I$ are families $(x_i)$ of integers indexed by I.

Addition and multiplication are defined componentwise:

$(x_i) = (y_i) = (z_i)$ where $z_i = x_i + y_i$

and $(x_i) (y_i) = (t_i)$ where $t_i = x_i y_i$

It is very easily verified becomes a ring."

In order to understand this example fully, I would very much appreciate an example of what is at work here showing clearly the nature of the families and their indices.

Peter
• Mar 8th 2013, 09:17 AM
jakncoke
Re: Ring based on families of integers - The Function Ring
Basically it fundamentally comes to the fact that if $I$ is the set of all functions from $I \to \mathbb{Z}$ then $I$ should be countable. Namely.
there exists a map from $I \to \mathbb{N}$.

adding 2 functions becomes adding their evaluation point wise. $\bar{f} + f = \bar{f}(i_n) + f(i_n)$ for all $\mathbb{N}$.

basically we can take the element in $I$ we attached a label "1" to and evaluate it there for both 2 functions, label "2" and evaluate it there,... for all natural numbers.

Again, we can do multiplication pointwise as well.