I'm teaching sequences at the moment. I've always put sequences in round brackets, for example $(1,2,3,4,5)$ is a sequence whose first member is $1$, whose second member is $2$, and so on. I've also always used round brackets to define a sequence in the following way: "Consider the sequence $(a_n)$ where $a_k = k^2+1$ for all $k \ge 1$." I would like to know if this is in standard usage.
On Wikipedia, they use the notation $\{a_n\}$ for a sequence. I thought "curly brackets" were reserved for sets where order in unimportant, e.g. the sets $\{1,2\}$ and $\{2,1\}$ are the same set. While in a sequence, the order does matter, e.g. $(1,2) \neq (2,1)$. Just like the points in the $xy$-plane differ.
To compound it even further, the course text does not use any brackets at all. For example, they say "Find the next term in the geometric sequence $1, 2, 4, 8,\ldots$
Of course I realise that we can use any notation we choose, provided we define it beforehand, but I'm interested to hear people's preferences and their own experiences.
Best Answer
I use $(a_n : n \in \mathbb{N})$ and I suppose $(a_n)_{n \in \mathbb{N}}$ would also be okay. I think $\langle a_n : n \in \mathbb{N}\rangle$ is good too, but I think $\langle a_n \rangle_{n \in \mathbb{N}}$ is fairly uncommon. I agree that it's best not to use curly braces except in contexts where it really is okay to forget about the order.
Personally I don't feel comfortable writing just $(a_n)$ (leaving $n$ as a free variable) for an infinite sequence; $a_n$ is a number, so $(a_n)$ is just a sequence of length one.