[Math] the correct notation for every nth term in a sequence

Question

How do I denote every nth term in a sequence? For example, if sequence $C$ contains:

$C = \{ 2, 5, 3, 6, 4, 5, …\}$

And sequence $Q$ contains every 4th term in C:

$Q = \{C_{4}, C_{8}, C_{12}…\}$

How to I express that using correct mathematical notation? In the example I have given above, $Q$ contains every 4th term – but what would be a general way to express nth term of a sequence?

My knowledge of mathematics is basic – perhaps there is a notation of which I do not know of. Thank you for your assistance!

Answer 1

Basically you can define $Q_n = C_{4n}$ for every integer $n≥1$.