Mathematical notation of set with $n+3$ members One of the math problems I have describes a set of numbers this way:

let there be a set A such that $A=\{{1,2,3,...,n+3}\}$.

I don't understand what the $n+3$ means and how the set actually looks like.

For this to have a meaning, $n$ must be already defined.
For instance if $n=3$,
$$A=\{1,2,\ldots,n+3\}=\{1,2,3,4,5,6\}.$$
The "$n+3$" means that you are enumerating all integers from $1$ to $n+3$.
Another notation (to avoid the dots) would be
$$A=\{i\}_{1\leq i\leq n+3}.$$
